And then there's something like Heavy Rain, which manages to be more of a "movie" than BttF, and remove all and any challenge from a game, and manages to sell millions of copies.
I don't think that Telltale games are killing the adventure genre. They were already dead. If anything, they are bringing them back to the mainstream. Regardless of the reason, BttF has been their most successful game yet in terms of sales.
The mechanics of AC have been expanded, yes, but they have also been incredibly dumbed down. You were talking about complexity of puzzles and that was what I was responding to: the fact that things have become easier in our generation. Apparently, nobody wants to become frustrated by having to think in a game. Exploration, that's another topic and I agree that AC has evolved the gameplay of PoP into a vast and very interesting open world. But, again, we were talking about difficulty.