What is a niche of Dragon Dance Mega Tyranitar? I honestly don't understand (or realize) what is so good about it. I know the bulk aids in setting up but...
...it needs Celesteela and Landorus-T gone or weakened, and its dual STAB is kinda unreliable. Stone Edge, as everyone knows, is lackluster in accuracy and PP, while Crunch doesn't hit too many things supereffectively. I feel like Mega Tyranitar is at most decent sweeper that requires tons of team support that can't do too much in return. I think it is outclassed by Mega Gyarados, which has Mold Breaker to defeat conventional physical walls, regular Gyarados / Dragonite / Salamence which can snowball and have access to Z-move, Zygarde which has insane coverage, and Mega Charizard X which has nasty Stealth Rock weakness but has much more limited counterplay.
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.