For some reason as soon as I get to the “end” of the game, I completely lose motivation to play. This happened with DOOM: Eternal, Sekiro, Bloodborne, Paper Mario TTYD, Cassette Beasts, Shovel Knight, Oblivion, Baldur’s Gate, God of War, Mass Effect… Pretty much every game I’ve ever played. It’s like the fun part was the journey, and I’ve seen everything the game has to offer, and I just don’t really enjoy the ending process. Feels like a formality at that point.
Terraria is over as soon as I beat the final boss despite having some extra stuff I could do. I’m playing it to fight bosses, I beat boss 1 to upgrade my gear so I can fight boss 2. Once I’ve beaten the final boss, what’s the point in upgrading my gear again?
Cassette Beasts was a game I played to 100% (as I do most creature collectors). I got this game to collect all the creatures, so when I beat the final boss, I played it for another 20 hours to grind out the end-game and obtain the elusive Magikrab.
Skyrim is a game where the entire main quest line is just like any other quest to me, and half the time I play Skyrim I don’t even touch it past the point where it allows dragons to spawn.
So that is to say, it depends on the game, and it also depends on what I’m there to do.