If throughout the playthrough you don't meet any old Pokemon, the gym leaders shouldn't have them imo. Post game when you have access to old Pokemon, and when you rebattle the gym leader, It's perfectly fine. I mean the purpose of a new gen is also to introduce new Pokemon, so imo it's fine if the new gym leaders showcase the new Pokemon and the types.
I'm fine with them using whatever Pokemon, new or old. I would certainly appreciate it if the Gym Leaders weren't still using only four Pokemon at the last gym. Especially when they have the exact same team as half of the trainers in their gym, but with one of the Pokemon fully evolved.