On the subject of Mew going up...
Honestly, I think you're vastly overestimating Mew, vyomov. Some time ago someone posted the exact same claim, actually, saying that Mew should be much higher ranked and how they were legitimately afraid of Mew getting banned in the future. Well, the thing is, Mew just isn't that good.
As others have already said, the best way to describe Mew is "a jack of all trades, a master of none." That's because, yes, Mew has an incredible diverse movepool that let's it due almost anything you want, and it has perfectly balanced stats to complement it. So why isn't it as good as you claim it to be? Well for one, Mew is a plain Psychic type, which many people agree as being an unfavorable type. Being weak to U-Turn and Pursuit and lacking in resistances really hinders Mew no matter what. The other main problem with Mew is that there's always something better. Sure, Mew can run a wide variety of sets ranging from Nasty Plot, Wall, Swords Dance, or Lead, but why use Mew when you can use something else? If you want a good Swords Dance sweeper, there are plenty of good options in OU already, including Terrakion, Breloom, Lucario, and Garchomp, all of which have their own great qualities such as speed, power, priority, great STAB coverage, solid typing, and even Spore in Breloom's case. Mew's balanced stats means that it lacks some of the raw power other common sweepers do, and its decent speed leaves it out-sped by quite a bit. Its typing doesn't give it great STAB moves to abuse or resistances to set up on, and its movepool, while fantastic, lacks some rare moves, like Breloom's Spore, that give other sweepers great advantages. The same can be said of the Nasty Plot set, although there's less competition for it, making it one of Mew's better sets. As a Wall, you have to again question why you're using Mew over something better. Sure, you could invest in its Special Defense and make it a special wall, but why do that when Jirachi or Celebi can do the same thing with better resistances? Why make Mew a lead set when you could use something faster and more powerful like Azelf? Also, you mention Mew has access to U-Turn and Volt Switch. Again, why use Mew as a pivot when Jirachi, Celebi, Rotom-W and Scizor can do the same thing?
The thing is, Mew just doesn't have a lot to make it stand out. Sure, it has incredible versatility, but that doesn't matter if it can't use it well. Now don't get me wrong, I think Mew is pretty good, actually. But we can't just say that Mew is unpredictable and therefore unbeatable. It may be "uncounterable" because it can run anything, but once you know what it's doing, it's fairly easy to handle. And don't forget, some stuff like Hydreigon are completely "uncounterable" with a single moveset, yet Hydreigon is B-Rank because its slow and has relatively bad typing. Same with Mew. Bottom line, Mew is versatile, but it has trouble distinguishing itself from the competition. B-Rank is perfectly fine for it, I think.
