Many well reviewed games have DLC that are not well received, doesn't keep up with the quality of the base game.
Or have DLC that are just blatant cash grabs, with just completion/milestone achievements.
Or you might like a base game, but you have no interest in a DLC.
Or, you finished a game to 100% and then later a DLC is released taking away your 100% completion.
All this can be a pain in the butt for completionist gamers.

This is why IMHO a DLC's achievements should be kept separate from the base game.
In order to 100% a game, the achievements for a DLC shouldn't count
and a DLC's Achievements should have a separate completion%.

I would even go one step further and separate multiplayer achievements from the single player campaign's. Because not everyone is into grinding the multiplayer or forced pvp aspects of a game, or even worse, the game is old, servers had shut down, and now it's impossible to get any of them.