Personally I’ve always found it kind of meaningless to 100% games. I don’t play a game to finish it, I play to have fun, and if I have to make ”do everything” a goal, then it usually starts feeling like a chore rather than me just having fun. I don’t usually like either looking up stuff just to do it, or meander about aimlessly. As long as I have fun I’m gonna play, but I’m not gonna make doing everything a goal, except for maybe a few outliers, like Bloodborne for example.
But I get that it’s different for everyone.
I think Achievements/Trophies can be a fun way to mark that a particular challenge has been completed, but they’re kind of lame in that I’d rather get that ”award” in-game than outside of it, and I’ve very rarely deliberately hunted them because like I said it’s kind of meaningless to me.
Though I don’t believe in backlogs either. I just play to have fun. Like reading a book, I can get back to them whenever I feel and be at the same spot I was before.
EDIT: Should say that it's happened that I've done everything there is to do in a game, but it's usually not been my "goal" or whatever. When I played Wind Waker as a kid I just played the game for a summer straight and just explored without thinking about doing it, and soon enough I had all the hearts and items and whatnot.
Felt like a true adventure.