I can see Luke eventually becoming the person we see in VIII, but it's hard to get there. He's gone through a lot in the decades since we'd seen him last, but we haven't seen any of it. One of the criticisms I see a lot about Luke in VIII is that there's "no way" he could've become that person. That bitter, that disillusioned, that disgusted with everything, that negative... in such a short time.
But wait, it hasn't been a short time, it's been a long time. It's because the timeline's a bit fucked in the sequel trilogy. After being "protectors of the galaxy" or whatever for hundreds of years, is 30 years long enough for the Jedi to have become legendary and almost forgotten? No, that doesn't make sense, and that's the real weakness, trying to make those two things take the same amount of time. But 30 years is long enough for Luke to have seen some shit, and done some shit, and 30 years is long enough to change a person a hell of a lot. Maybe people were thinking that he was just off somewhere, travelling the galaxy, enjoying his retirement/exile/whatever. And then we find out through flashbacks (which weakens the overall effect) what happened.
Anyway, TLJ still has its problems and some will always bug me, but I might be coming around on the Luke thing.