For example, I'm sure that Justin Sun is getting ready to leave the project, since the amount of TRX that authors receive for their posts has been gradually decreasing recently.

Good point, and you might be right. I hadn't thought about that possibility. My thinking had been that the fund might just be running out of tokens. Around the time that the TRX rewards started, I seem to remember that steemit had transferred something like 22 million STEEM to one of the exchanges. I had always assumed that that transfer was what funded TRX Integration, and that TRX rewards would eventually run out. If so, the repeated reductions might just be intended to extend the fund's life.