I think that banning Pokémon just because their BST is 600+ is just retarded. Those guys listed may have high stats, but they aren't over-centralizing the metagame. If you ban them, then other Pokémon will take their place. Those Pokémon will then start to be so good that another thread like this will pop up complaining about those Pokémon being over-centralized, which results in more bannings, and so on. This vicious cycle will gradually spell the end of competitive Pokémon battling as we know it.
Restricting them is also unneccesary as far as competitive battling goes. Competitive Pokémon battling is a sport which was meant to be a skill-based game, with a simple tier system. People should be allowed to choose whatever they want on their teams. Sure, a restriction will get players to use a wide range of BST's, but for the most part, if people want to use all 600 BST, by all means, let them.
Actually, no.
Banning the base 600's has little impact on making other things more broken.
Sandstream will still be provided by Hippowdon, so that strategy remains viable.
Much as we will all miss Metagross and Jirachi, Bronzong is still around, and Steelix and Forry could provide Steel STAB for those who really want it.
Cresselia gone? Hi Uxie and Mesprit, were ya been? Regi's still fulfill walling needs.
Celebi and Shaymin? Sad to see you go. Good thing we still have Blissey, Miltank, Vileplume, and Meganium.
Heatran. Well, you don't really have any comparisons, but there's still plenty of awesome fire types around.
Chompy and Mence gone? Good riddance. Flygon and Altaria make a comeback.
About the only thing removing the base 600s will do is decrease the number of uncounterable sweepers. Every strategy that was viable before them will remain, although you won't be seeing boosted Outrage anymore. You still have the Trios to troll around with, and its hardly as if all the heavy hitters have been removed. There are literally a dozen pokemon with 130+ Base attack, even after removing Chompy, Groos, and Mence.
Cresselia, Celebi, and Jirachi were decent defensively no doubt, but with the biggest hitters in OU gone, the inferior walls should suffice. None of the base 600s counter Heracross as well as Gliscor and Weezing do, and none of them are particularly lethal to Gyarados.
It would certainly be a different metagame, but by no means an inferior or less balanced one.