Not sure why this got ignored. This viability thread is keeping its mindset way too far in the past, as evidenced by Latios and Latias being in A+. Neither of them are top tier offensive threats anymore this gen. Heatran, Tyranitar, Ferrothorn, and Scizor are arguably even more popular than before, and since the metagame isn't revolved around special offensive threats anymore, both of them have lost a large amount of their switch-ins. Greninja is the only premier special offensive threat and he outspeeds both of them, and packs U-turn if he can't seal the OHKO. Sure they can switch in on Rotom-W, but if their switch-in is predicted with a Volt Switch, you can be sure they're going to go to a Genesect that is going to immediately force them out again. Speaking of which, new threats such as Genesect, Excadrill, Talonflame, and especially Aegislash all make life harder when it comes to spamming Draco Meteors. And that's assuming they even get to pump out a Draco Meteor, since U-turn and Dark/Ghost-type moves are everywhere this gen.
In addition to the metagame being much harsher for them, pretty much all of their offensive options from last gen have been hit by the nerf bat. Dragon Pulse, Draco Meteor, Thunderbolt, Surf, Ice Beam (lol), even HP Fire have all received nerfs. And this isn't even taking into account Fairies, which while not insurmountable, definitely make their lives much harder.
The only thing they gained this generation was the buffed Defog, but there is no way that is substantial enough to keep them in A+. They need to be bumped down, I'd say Latios in A- and Latias in B+. Latias is having a harder time in this meta than her brother, since the current offensive momentum neuters the hell out of her CM set. Sad because her CM set was such a potent end-game sweeper last gen.