So while I upvoted because it is clearly a personal opinion, adding achievements to games over time can impact people in a couple of ways. I sometimes do them for fun, but know some people who are dedicated to completing sll achievements for games they love.

Someone who has already hit 100% achievements would lose that 100%. A lot of people care about stuff like that.

If the achievement is time locked, someone who starts later cannot 100% the achievements. I have some achievements in a game I played that can never be achieved now because the conditions needed for it to be met no longer exist.

There are some people that like time locked and unobtainable achievements, so it is a balance between people who want ongoing achievements and those that want a specific list to work through at whatever pace they choose to play.