The more content the system is filled with, the less the average reward for each creator, unless there are many whale-like investors.

I don't think this is the case. Hivers leave in the same proportion basically every month since the fork. The rewards per post adjust substantially, and if anything are actually higher when the larger numbers of people left historically.

https://peakd.com/hive-133987/@demotruk/do-hivers-leave-because-of-lack-of-rewards