Mewtwo doesn't have a "consistent" counter meaning that there is no Pokemon that can counter every one of Mewtwo's movesets. That much is true. But the same could be said about Tyranitar who in 386 has at least 3 viable movesets, each of them countering those Pokemon that counter another of Tyranitar's moveset.
For example: the "original" Tyranitar moveset had Dragon Dance, Earthquake, Rockslide and either Taunt or Substitute. This moveset generally used to be countered with Claydol, Donphan, Swampert, Flygon, Milotic and Suicune. But people started to mix in different movesets for Tyranitar, one which uses Substitute, Focus Punch, Crunch and Thunderbolt - this moveset was more based on special attack power and countered literally all of the Pokemon mentioned before at least to the extent that you wouldn't switch them in against that Tyranitar. Now you've had to find a direct counter to
that moveset, which - somewhat surprisingly - turned out to be Heracross. But only if the Tyranitar decided to not use Flamethrower over Thunderbolt, in which case a water tank is the better choice. But how do you know? There is no consistent counter to that - the most reliable thing you have is switching in Dugtrio but that only works if Tyranitar doesn't have an active Substitute while at the same time one of your Pokemon has just died [because that's the only situation in which you can actually switch it in against Tyranitar]. So essentially, it becomes a guessing game between countering the Dragon Dancer moveset and the various SubPunch / mixed sweeper sets. The only Pokemon that legit can be switched into all of Tyrantiars viable movesets is Swampert and even that is not a good idea in all situations.
Cases like that exist for Jirachi, who can change up his moveset to circumvent his counters as well [which are Claydol and Steelix] and even Pokemon like Salamence or the underrated Dragonite could do that. Most people would just not think outside of the box and use standard movesets without understanding the how and why of this game. That's why even in a more advanced metagame people would either resort to SkarmBliss or complain about it.
You're right for the most part but you overemphasize the importance of speed in this case. It's about being able to generally outlive Psychic via resistances / immunity and being able to hit back hard enough to reliably destroy Mewtwo's substitute. This is easier achieved than a lot of people think. Actually, every single of Mewtwo's sets has a clear counter. Psychic, Calm Mind, Substitute are basically a must-have for Mewto in 386 so its counter depends on which he uses as his 4th move. Flamethrower?Beaten by Tyranitar. Ice Beam? Beaten by Metagross, even Jirachi could be iffy. Thunderbolt? Beaten by Claydol.
It should have at least been tried.
eachbrawl: 
