Golden age of forums was more than a decade ago, so no wonder that bitcointalk userbase keep decreasing as I've seen the same thing on the other forums I'm using; old users quitting and new ones are attracted by other platforms.
The user base will also drop further if the altcoin (bounty) field no longer profitable anything for its participant. I can't imagine how this forum will have any user appeal if altcoins (bounty) are removed.
OP, I feel if the benchmark of forum user base is seen from the cycle of senders and recipients of merit then I'm not sure it will be accurate enough because many forum users ignore the merit system and even they are not interested in getting involved as senders nor do they want to receive if we pay attention to the quality of their post. There are thousand of post on cryptocurrency alternative discussion board that do not meet quality standard and most of the people involved are bounty hunter. They're still active, but don't appear to be users interested in engaging either as senders or recipients of merit. In addition, the merit system also has an influence on the current user base of the forum.