@atomicpoet dwindling MAUs are reported in all stats (including the-federation and Fediverse observer). The actual numbers may be debatable, but I doubt you could make the argument against the qualitative assessment. Even if the stats are missing 10K instances with an avg of 100 users each, 100% active, it just means that instead of 2.5M active accounts out 10M you have 3.5M active accounts out of 11M —not that much of a difference, and with a generous overestimate.
@atomicpoet much of the recent growth has been reported by mastodon.social (see @tchambers per-instance decomposition). If people just registering count as active, and the MAUs reported by these instances is going down, it means more accounts are going inactive than new ones are being created. This is compounded by an increase in spam accounts, that get blocked more or less quickly, abd probably instance migration (does the old account still count as existing?)