They could stop sponsored segments, but they couldn't stop creators and users from going to other platforms where they allow sponsored segments. They have far less control than e.g. Apple with the app store (where they literally can stop other app stores from ever coming into being, barring regulation that changes that).
If youtube stopped allowing sponsored segments that puts pressure on the market to produce such a thing. Even now creators are trying to come up with alternatives. Nothing has panned out, but something like stopping sponsored segments could very well tip a large number of people who want to get paid to find another way to get paid.
So what, even despite high-profile creators such as Practical Engineering constantly pushing for Nebula (the largest of them), it's still a fraction of their YouTube following.
