A mix of bias and ignorance, nothing's going to happen if you don't open people's eyes.
That said, I genuinely don't think of the SH props are best in the world at the moment. Australia's props obviously aren't, although better than given credit for by some. I don't think the Kiwi props are all that, but that might be because NZ scrum to create platforms and not to milk penalties, which obviously doesn't stick in the mind. I think the South African props are somewhat raw in terms of technique and achieve mainly through power, and they are picked for power to perform in the loose. My limited viewing suggests there's better scrummagers not getting Bok caps, although I may well be wrong. And the Argie props are the wrong age.
Argentina had a golden generation of props previously. Reggiardo, Hasan and Roncero etc at their peak were some of the best of their generation. They created the scrum reputation, although the current Argentine props really don't have much depth behind Figallo (a loosehead playing tighthead for them) and Ayerza. New Zealand's props aren't the world's best imo, they are adequate, but not the world's best.
There's a fairly solid body of evidence suggesting this is no longer true for a great many coaches. I'd have personally said Gethin Jenkins starting ahead of Paul James was an example, I'd have James as the better scrummager, although I may be wrong. Still plenty of other examples abound, a great many in national teams to boot; I've already said I think the Saffas do it, the Irish certainly pick for loose ability first, the English propping hierarchy underrates a lot of our best scrummagers imo, whoever's in charge at Sale doesn't appear to care, the French surely have better scrummagers than Debaty... the list can go on for a long time, adequate/good at scrum time and outstanding elsewhere is now acceptable in modern rugby. Whether it should or not is a matter of opinion, and personally the greatest part of the debate over Jones and Cole that is not directly tied to nationality.
With matches nowadays being able to be won and swung the way of the team with the dominant scrum, any team hoping to become the best must have at least an adequate scrum. I remember a Canada vs Italy match this year, where Canada were the better team in the loose, but were dominated in the scrum to the extent that they could hardly win their own ball and that lost them the game as Italy collected points from it.
The minimum required is a scrum that can hold it's own and not concede too many penalties. But especially in tight matches where sides are evenly matched, if one team can dominate scrum it can make a big difference to the scoreline.
Also from those props you named, Gethin Jenkins isn't as good as James, but he was a good scrummager, it is only recently he seems to have lost it. For example the clip below shows Jenkins in his second cap dominating New Zealand, and in 2012 he was part of a Welsh scrum which got the better of England. I don't think he's ever been as non-scrummaging prop as made out, and just these last 6 months he has struggled at Toulon and for Wales against Samoa. I wonder whether his confidence was hurt by those few months last season next to Scott Andrews when Andrews was getting shunted about by all comers.
Also Jenkins had Jones alongside, so Wales could afford not to have to select James for most of the time. And James hasn't always been the force he has become over the past few years, for a long while Duncan Jones until he suffered some big injuries was the preferred choice at the Ospreys and ahead of James with Wales.
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Whilst Debaty isn't a great scrummager, he rarely starts for Clermont. He is seen very much as an impact prop. The stats back this up, he has never started more than 7 games in a season for Clermont (
http://www.itsrugby.fr/joueur_468.html), and that's from a 32 game season + playoffs and Domingo has suffered a couple of serious injuries in that time. Clermont normally started Lionel Faure or Chaume ahead of him even though Debaty may get ahead to be on the bench behind Domingo. When he did start for Clermont against Leicester last season and Leinster this season, it was a disaster scrum wise.
Considering he's 31, and has 11 France caps and just 2 starts. It would seem he is simply an impact carrying prop for them too. Not good enough scrummager to be a starter.
Sale have some awful scrummagers in their side and they aren't adequate imo, I don't know whether relegation will make the coaches realise this. I saw they conceded another penalty try against Montpellier in the H Cup, I saw Gloucester amongst others take their scrum to pieces as well. Not surprising though considering the personnel they have there. Few would consider Tony Buckley a good prop, even though he is quite handy in the loose.
I'd still rate Tony Woodcock and Owen Franks as two of the best props in the world. Steenkamp was magnificant in 2010 as some people mention - while the Beast was very good until his heart problem arose. All these guys could well be rated around the best - I'd take Owen Franks at TH over Adam Jones or Dan Cole (although they are quite similar - I just think Franks is more physical). The problem with the All Blacks scrum at the moment is that for a start we lost Brad Thorn and for another there seems to be a strange bias when viewing the games that the AB scrum isn't one of the best. It actually takes the players going over seas to be rated. No one seemed to rate big John Afoa before he went to Ulster - now some would argue he is the form prop in Europe if not at least Ireland.
The New Zealand scrum impressed me in 2011, but less so in 2012. Ireland managed to get the upperhand at times with Declan Fitzpatrick on debut. I do rate Owen Franks, but not as the world's best.
Also Adam Jones did a job on the All Black scrum in 2010. One of his scrums gave Wales the momentum for a try.
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Also you mention All Black props going overseas, and you're right Afoa has done incredibly well with Ulster (so well you wonder why he didn't go further with the All Blacks), but it's not all of their props go and be a success though. John Schwalger and Neemia Tialata for example have both been duds in the Top 14. Campbell Johnstone got destroyed in the Heineken Cup final with Biarritz.
Appreciate that, but at the moment we're in some kind of a propping crisis. Apart Nicolas Mas (who, yes, has been a bit ignored in this thread, except the OP), there's a huge void at TH. And,at LH, Domingo hasn't picked up form, Debaty is not really a good scrummager (one hell of a ball carrier though), Barcella has never been back to form since 2010, and the others (Ben Arous, Forestier, Chaume) are one international career short.
For me, the best prop in the world is Owen Franks. Nicolas Mas, Dan Cole and Adam Jones come behind, quite close one to the other imo.
What about Cencus Johnston?
He has delivered in a Heineken Cup final and a Top 14 final (as big a matches as there are for a Samoan) for Toulouse with dominant performances in the scrum (albeit with perhaps one the best scrummaging hookers in Servat). And also began to dominate Leicester (with Cole) in the second half in the first match of this season's Heineken Cup, and against Wales in November where Wales' scrum disintegrated.
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Which is why I don't think Cole is the best in the world yet, he hasn't delivered on the biggest stage in the big matches against big teams yet. Johnston has delivered in finals against big sides like Toulon, Biarritz and Leicester.
Also Johnston dominates other scrums like Toulon in the Top 14 final, every big match I see Cole such as the most recent ones with the Ospreys and Toulouse he seems to flop 90% of the time. He sometimes gets penalties from it (scrum penalties were pretty evenly split in both matches I referred to) but he doesn't seem to dominate and push the opposition backwards in most of these big games. In the first Ospreys match, Bevington was yellow carded, but he wasn't going backwards, it was after a scrum had just flopped.
Also Cencus Johnston is one of the best ball carrying props about, and that type who can make yards like this. For all the talk of Cole at the breakdown, I don't think he's that good ball carrying in the loose, and I remember him costing England a certain try against Fiji at one point.
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