I knew I shouldn't have used the word "never", although I was thinking that only "deleted" accounts would drop the total count. That's also a good point.
But it still seems unlikely that there will be a LOT or cases where the total number drops by a proportionately large number, unless something really bad happens (or there's some big initiative at some point to remove inactive or not accounts)
The more important stat - the one that people would be much more likely to be talking about seeing a "decline" in, in cases like this - is "active" users.
I was thinking more about the frequent cases of Mastodon instances shutting down because the admins got either tired of dealing with moderation or unable to afford the increasing operational costs. Seems like every week we get at least one instance announcing they will close up.