Shouldn’t Ubuntu be more “mainstream” than debian?
…and less “mainstream” than Windows?
That’s the Windows 7 logo. Windows 10 and 11 are in the upper right corner.
Why are there multiple windows logos? You don’t see Fedora 42 and 43 on the plots nor the different versions of any other distro.
Not if you’re including servers.

In that case Debian, Ubuntu, and Windows server should absolutely dwarf OSX…

That said, I personally wouldn’t consider “server use” to be “mainstream”… To me install-base does not equate to “mainstream”.

even in just the desktop space, macos only holds any significant share on western europe, usa, canada and australia