Either one or both works.
Mine is completing the Pokedex in the original Pokemon games. All you get is just Professor Oak giving you a wink and a small few second cut scene. And a congratulations text. Imagine spending all of your time then, getting all 151 and even 252 pokemon just for that? Yeah no thanks, I never completed the pokedex.
Going the Joja-Route in Stardew Valley. I say this mainly because, it is what you make of it. You forfeit being able to complete the Community Center by earning things, when you sign your soul away to Joja. What I would’ve liked is seeing Pierre go out of business completely. I just think that would add a route of depth in the game where you have to make ends meet through Joja because Pierre is permanently closed.
But, that doesn’t happen, he’ll still be in business despite his depression about Joja running things. Kindof ruins the whole concept of doing it for the achievements even.
I’ll only answer the first one.
Achievement systems full stop. People who value completion through achievement systems are fucking uncreative persons who need to find a different hobby or reconsider why they enjoy theirs. From a dev standpoint it’s just a way of lazily padding a game.
I’m not talking about completionists of actual game content like collecting all the stars in a Mario game, or catching the 151 pokemon, but moreso the “silver trophy” for killing 2000 grunts or whatever bs achievement ideas they decided to arbitrarily create. You’re diluting the art form.