Yup, I've had this same, exact idea plenty of times before, and I don't believe it would be so bad.
There are currently over seven-hundred Pokémon out there, with more being continuously added. Each and every time they do add, it becomes even more tough to complete PokéDex and whatnot, which, I feel, is something that most of us want to achieve someday, or have at least wanted. Back when generation three was still active, I thought that the amount of Pokémon they had was unbelievable, and even so, I've managed to capture a lot of them. However, if I try to perform any of this now, it would consume ten-times more time and energy, I feel.
Pokémon are like animals, so in some ways, it does make sense to have a variety, but I still believe that it would be worthwhile had they limited themselves every now and then. If they have design ideas and wish to put them into work, there are plenty of other ways to actually do it, such as creating mega-evolutions, secondary forms, and so forth. Speaking of mega-evolutions, I'm glad that they've introduced such a feature; whether it be intentional or not, they'd likely be slowed down by it.
There are countless amount of Pokémon in which they could work with, so if they were to introduce a game without any new Pokémon at all (except for megas and whatnot), it would be great. There are many of them which some fans don't know about, such as Relicanth, and if they were to be used more, it wouldn't do harm in any way, I feel.