On the other hand, isn't the FreeBSD user base shrinking and its former users going to Linux?
As someone who very reluctantly fits that description, yes but no. I don't love FreeBSD any less or want to use it any less than anytime in the last 20 years, nor do I like Linux any more than at any time in the last 20 years (less, if anything). But NixOS is the "killer app" that has me on Linux instead of FreeBSD. If either FreeBSD gets a really good NixOS-like setup, or Linux-based NixOS becomes unmaintained, I'm happily going back to FreeBSD again, it has always been excellent and stable to me.
*BSD is dying! You don’t have to be Kreshkin…
But seriously, if one counts macOS and iOS as FreeBSD users, there are more than ever. Of course that means counting Android and Steam as Linux OSes, in which case Linux users still greatly outnumber FreeBSD users.
No. As enshittification encroaches onto Linux the user base is moving the other way. To their benefit, I might add. With ZFS on root in FreeBSD it's a no-brainer.
Linux is feeling more and more like a bunch of random tools thrown together as opposed to a complete OS designed to work as a whole.
As a 25 year Linux user (for work and at home), I've been experimenting with FreeBSD in the last year or so and I've found its simplicity refreshing. Maybe I'm swimming against the current, but I'm sure there are dozens of us!