I understand that initially they did try to clean up the ruck, but it was too hard to officiate, so they decided it was better just to scrap it in favour of a play the ball. And yes, there was not six tackle rule originally.
As to the second part of what you say, that's entirely a matter of perspective and it's one I can assure you almost every League fan would disagree with. Do me a favour and get a game of league and union side by side and tell me which recycles the ball quicker, a ruck or a play the ball. Apart from an uncontested ruck, the play the ball wins every time. So in what sense is it any more stop start than a ruck? Both involve the tackled player hitting the deck and then attempting to release the ball to another player, it's just that in League you're not allowed to interfere with the process.
As to your complaint that players will try and play the ball slowly in the final minutes of a close game to eat up the clock, well they do the same thing in Union by just bashing it up with pick and goes so that they've got enough players near each ruck to protect possession. Heck, I've seen three games this season where the Waratahs have done just that and it ****** me off just as much. Now, let me emphasise that I prefer rucking to play the balls, but I don't think the argument that the play the ball slows or stops the game has any legs in how it actually plays out.
Lastly, AFL is played in 4 quarters and has 2 goals in effect; one actual goal worth 6 points and a concessional "behind" goal for kicks that miss the actual goal within a slightly smaller margin of it. But fair enough, I don't really know too much about Gaelic Football, only that we play some mickey mouse half-half game with you guys so the AFL can pretend it has an international season. But the CFL and NFL is a pretty good comparison all the same.