Hear me out: Ben 23 cycled around his aliens to stoke demand and retire them every so often, showing off new ones and what not. The execs of this reboot are doing the same thing....and because I can only assume it based on populaity that is going to create a problem.
Namely that certain aliens are never going to leave, and while the idea of limiting Ben's alien count does have advantages to the sprawling list Omniverse Ben had, having multiple aliens with the same basic role is going to squander the strengths of 10 aliens oly.
(Fourarms isn't going anywhere, and Rath is probably safe. That's two physical power aliens assuming that Humungosaur doesn't stay around as the least popular of the three).
Ben 23 sorta "fails" because he's limiting himself on purpose, making him cockier. Reboot Ben on the other hand acknowledges that the limit is a hindrance, he's not gonna "stoke his demand" (especially not with an alive Max and a present Gwen around).
Plus, that idea was there since OS. They dropped it when they decided to add more aliens, but it was thought of a while ago.
When Ben 10 Alien Force came out, they "recycled" the aliens that he had available as well. From the cartoon point of view, it was the Omnitrix giving Ben a random assortment of aliens to choose from out of the 1 million plus available.
From the producer's point of view, new aliens means new merchandise (toys) to sell. And since the old aliens still show up from time to time, those toys will continue to sell. So, adding more aliens means adding more toys.
Interestingly, Ben 23 can get away with that with sets of 23. He can have fan-favorite mainstays like Handy Man and Dino-Mighty and still have room for unique aliens like Vomit Man, Muck-A-Muck, and Plug Man. They can't do that with Reboot Ben and his sets of ten, as it would get cluttered.