The gap of 30 years makes it perfectly okay for Luke to be anyone. The end of TFA doesn't push Luke to the dark side, but the idea that makes me think he could be bad is because of the need for something that rivals the twist of TESB, plus it would give real point to the sequel trilogy.
There is precedent; John Connor became the Terminator, despite having been played by Christian Bale and being a central good guy in 3 previous movies. And the character Jim Phelps was flipped bad for the first M:I film.
The end of TFA does render the sequel trilogy pointless: why should we believe that Snoke is so deadly that the end of Starkiller base is meaningless?
TFA is like the original Star Wars (a self-contained story), except that there's some margin of sequel-bait.
In practice, what I think will happen is that something bad will happen on Ach-to, at the end of Episode VIII, assuming the movie ends on that planet.