I mean, Eevee and Meowth are rather important in the games and anime at this point, so there are more options. I can understand not wanting to make the roster too huge with them, though. I think one or two new ones per game, max, nowadays is fine. And I'd be fine they didn't add any new ones in any game. I don't really believe in a series needing a guaranteed slot. Not that I disagree with the idea of saving a slot for Pokemon's next gen, either. As long as the character is able to stand out and be fun, it's worth it. Basically, let's say the next game comes around Gen IX's time. If Sakurai sets a slot aside for the character, but no Pokemon from that gen interests him, it's not an issue whatsoever that he either goes with an idea from a previous Gen he likes or simply doesn't add a Pokemon in that one game. That said, it wouldn't even be hard finding an interesting Pokemon in any such Gen. Also, I don't think the "deserving" argument really is a good one either. There's many worth promoting. It doesn't mean they should just simply do it cause it's Pokemon. Series balance is still something that should be considered. Just like a smaller series is still nice to give another character, even if the franchise is dead/on hiatus(like F-Zero). Neither are bad ways to go about it.
Albeit, it does help that Pokemon all get in for various reasons. None have so far gotten in just for the sake of promotion at this point. Though it could happen for DLC, but the first time we ever got a single character just for the sake of advertisement, but I doubt it even then. Choosing a Gen VIII Pokemon also means figuring out why they're interesting enough, giving more than just "advertisement" as the reason. Greninja, for instance, wasn't just chosen because it was from Gen VI, it was a character Sakurai immediately envisioned in his head of how the character works. The timing of 4 and X/Y releasing close to each other didn't play that much of a role anyway. He just thought he should save a slot for a Pokemon of the upcoming Generation because it's a massively popular series and is brim with potential. Incineroar is the same, but his games were already done with before Ultimate even came out, making him even less of an advertisement pick at that point than just being there because he wanted a Gen VII Pokemon(not to advertisement either, which at least could be a decent argument for Greninja due to the right timing of release dates), but the other key factor is that our fire tiger has Wrestling as his mechanic. This is a key thing that allowed Sakurai to choose him over Decidueye, the only other considered one(who was going to be focused on projectiles, so basically, implied to be archer-focused. The funny part is this is why people wanted Decidueye in, since we don't have a regular archer character, just people with a few bow-related moves).
I did notice how since 4, we weren't getting at least two all new Pokemon per game(which coincidentally were also two literal Pokemon slots per game, though Pokemon Trainer was really 3 characters in one). And that's okay. I felt with that many Pokemon already in the series, cutting down to 1 max per game wasn't a bad idea. This allows other series to flourish a little.