Many incremental games can be played for a very long time.

I'm going to repeat myself here and also miss the topic a bit - because I'm not a fan of post-game content, endless modes or even replays - but I know of no games that can be played longer than Neverwinter Nights and Forgotten Realms Unlimited Adventures due to the wealth of quality custom content available for them, which should last you for years (just not with the same character(s) in the same playthrough, but you will never have to play the same story twice, there is always something new to discover).

Other than that, more in line with your original question, I guess Dragon's Dogma has a longer post-game mode? But you already knew that ;) - and personally I got bored of it very quickly.

Dragon Quest 11 hides half of the game behind post game or a 'fake ending' if you want to call it that.

Technically any mobile game or gaas would qualify since they add updates and new content to keep players engaged although the quality and money you spend would raise some eyebrows. These games also tend to have an initial campaign so everything else could be considered post-game.

But for older single player games, I think games like Devil May Cry which encourage replayability to improve skill and competence through each run for a higher score do post-game well.

No Man's Sky? I mean 18,446,744,073,709,551,616 planets, endlessly generated missions, crafting/building/composing songs...

Tales of Maj'Eyal is a roguelike that includes an Infinite Dungeon which becomes accessible once you finish the main plot - once you enter though, you can't leave. And for those who care about such things, you can collect achievements for getting as far as level 500.

There are currently 3 official expansions available (and a fourth in development) and, being open-source, there are lots of third party addons (extra content, UI changes, rules modifications, etc).

Pretty much any game making heavy use of procedural/random content generation would be worth including though. Aside from the (mostly) RPGs mentioned previously, strategy games with random maps (Civilization series, Heroes of Might and Magic series, Age of Wonders series) should be worth considering.

Tetris?