Looks like I have embarassed myself a little bit... also I meant only Zard X has more narrowed counterplay, not every 'mons I have mentioned...The fact that Mega Tyranitar has so much natural bulk, in conjunction with Sand Storm making it insane on the special end, lets it set up much more than any of the alternatives. It has a pretty much endless movepool, letting it potentially run both STAB moves, Ice Punch, Fire Punch, and Earthquake depending on what you feel is best on your team. It does indeed need things to be weakened to straight up sweep, but the same can be said for just about every win condition -- Knock Off, Stealth Rock, Sand stream itself, etc. all play into this, so yea while you will have to play the game out, Mega Tyranitar is a fine end-game win condition and while it does not stand out a ton, it is viable and there is a decent amount of appeal to use it.
Also, I do not see how things you mentioned such as lack of PP on Stone Edge or super effective coverage on Crunch when you don't truly need either of these a ton of the time, especially given the nature of any sweeper and the coverage Tyranitar has. I think that saying Gyarados, Dragonite, and Salamence have less counterplay is pretty ridiculous, by the way, as they are checked well by the same things you mentioned above, which barely even check Mega Tyranitar, and other Pokemon. Zard-X is pretty poor right now due to Stealth Rock being so annoying and it requiring far more direct support than Mega Tyranitar. Finally, Mega Gyarados is simply not as good a win condition against offensive teams as most scarf users still RK it at +1 and it lacks the bulk of Mega Tyranitar while it no longer needs to really break through stall w/ Mold Breaker Dragon Dance as stall has taken a big hit w/ the banning of Arena Trap. All in all, Dragon Dance Mega Tyranitar is a very passable set and MTar is one of the best DDers in the tier.
Yeah I did some calcs with Tyranitar bulk and it is quite phenomenal. Anyways thanks for writing semi-essay length response