I don't doubt that it's a much better game now, but while playing, my heart no longer felt particularly into the game, and sometimes that's what kills an interest more than than the quality of the game.
Makes sense to me. I briefly played and then quit D3 a couple of times and didn't really "come back" until the Necromancer got added. I still like Path of Exile, style and concept-wise, but my heart isn't in actually playing it, because it's become such a speedy screen-clear kind of game, so I haven't really played the last few seasons (but am excited for PoE2).
I imagine D4 will be around for quite a few more years unless it suddenly stops making money. That seems very unlikely given they are now progressing in a much more positive direction and given the sheer number of people I see around who are physically showing that they bought the $90 version of the expansion (which is normally $40), or wearing cosmetic armour that costs $25+.
And because it'll be around, and isn't subscription or whatever, you can always dip back in. Given that the base game + expansion pack is $70 just like the base game was, I imagine future expansions will package all previous expansions with them too (for $40).
Re: Leah 2.0, I doubt it, I think they probably learned from the the pushback on that (D4's story largely avoids the pitfalls D3 had, albeit it's still a Diablo story). More likely we catch up with her in either this expansion or the next before she becomes a demon, but the stone will end up getting taken from her or handed off or something.
(The only way I see D4 getting "taken out" is if PoE2 is drastically better - and whilst it definitely has better art design, and will undoubtedly have a smarter story, I am skeptical that the gameplay and performance will be better enough to convince people to change over to it. Not impossible though.)