This, 100%.
Adding 5 more levels would just invalidate a lot of previous work on pets, adding new trainers and new pets is a lot better and adding a lot more depth than just having us level all those pets another 5 levels.
I have around 100 level 25 pets (most of them leveled from level 1), and I certainly would much rather level up brand new ones than just keep at it with the existing ones.