Tyranitar being S rank is a ridiculous idea. It's nowhere near as dominant as the S Pokemon we have already, and it gets destroyed by a significant chunk of the metagame without nearly providing support on the level that Politoed or Deoxys-D does. Frankly, Tyranitar is only so good because of the presence of Politoed and Ninetales - without the presence of other weather there would be far less reason to run Tyranitar. For this reason, it should remain A tier.
Tyrannitar's merits as a pokemon are supplementary to the sand it provides; it is most certainly a better poke than poli or ninetalrs, while still providing similar support.
And as mentioned, it serves as a nice way to deal with other whethers, simply by chaanging them. It can double sandslash+stoutland's speed, it can make lando extremely strong, and it can provide a very nice bulk boost to pokes like terrakion (ie... Gene ( in that meta) could no longer ko, it can now take dcalds, etc) plus the residual damage