Honestly as much as people bitch about the Dexcut it does provide the game with some unique opportunities in competitive because it doesn't just stay stagnant for the entire gen and DLC and updated can reintroduce old mons with potentially new learnsets that totally turn the meta on its head and allow for new strategies.
If every Pokemon was in the game from the start you'd basically have a solved game from the start of the gen but this way you get a shake up every year or so that stops it from becoming too mundane.