I did finish PoE so many times I lost track of it. BUT the OP has truth in it: in my first play through I also nearly stopped right after reaching Twin Elms. There is a sudden drop in motivation, a break in the game flow, whatever.
What kept me going was this forum. More precisely the character builds subforum.
It's fun to build powerful, quirky or even game breaking characters and try them out, then posting them here and discuss. Actually the last part is most fun I guess.
So I would say what kept me going all this time is the forum and not the game itself. Although I like it a lot, too.
100% the same
1 addition: playing mainly as a solo player in potd i have to say that i like much more the First part of the game, where every fight is diverse and you didn't have many option. In late game all fights become "you vs the horde", when you have to fight costantly 10+ enemies, their type and abilities matter much less, and the fight is reduced to survive alpha strykes/ do massive aoe DMG until you remain the only one alive. I would have liked much more some fights in end game against only 2-3 very powerful enemies, instead of constant floks of lower ones. The grey bear in wm1 can be a rare example of sigle Monster fight