That's fair enough but what then about these players playing for other countries in U20 or national schools teams earlier in their carreers? IMO DON"T play for the country you are not going to turn out for as you are denying guys who want too and are willing to make a commitment that chance. You are old enough to decide where your loyalties/identity lies. To get back to my point U20 is old enough to make a commitment IMO; how many guys around that age are turning out for senior sides in recent years! I can't see why they shouldn't be 'locked in.
Fair enough line of argument if it goes for everyone. Personally I'm not for it - I want players to go and take the best opportunities available to them at that age without worrying too much about what it means for the future. It makes for better rugby for me to watch. Nor do I want players being called in just to give them a cap. From a purely selfish point of view, I should probably be in favour of this: England get a reasonable influx of players from SA and NZ, but in turn we leak players to our neighbours, and right now I feel we do worse out of it as a rule. But I'm not that fussed in that sense. Player movement will not make or break England's international future, nor is U20 rugby a massive indispensable tool in player development. I just want lots of good rugby to watch, and what we've got enables that.