First of all Mega Garchomp isn't broken yet. We don't know the details about it. Assuming it's a Kyurem-B with better typing, it'll definitely be OU, no question about it. I think you're underestimating the power items have on Pokemon. The only Megas I can see being Ubers, are Mewtwo (both forms), and Blaziken. Mega Lucario might be Ubers, but we can't tell for sure. Megas, although a dramatic change to the meta, won't be as big of a threat as you're making them out to be. The Megas for lower tier pokemon will probably move up a tier or two, like Mawile and Ampharos, while their original counterparts will stay in place.