For Roblox I haven't seen the returns getting smaller. I've had a bunch of smallish games there for a few years, and it has been stable.

While there is a ridiculous amount of competition, so far it has been offset by platform expansion. When I started in 2020 the whole platform had about 30M daily active users. Now over 110M. Maybe my share of plays has shrunk, but it's now from a much bigger pie.

(I don't know if this holds true generally or if my games have somehow persisted better than the average game)