2001 was immediately post dotcom crash and so all the people that had bought into the Sun "the network is the computer" were tossing out expensive E4Ks, and getting cheap intel servers to survive.

HP-UX and AIX were already legacy.

Linux 2.4 was when it hit critical mass because of the publicity of the dotcom boom and it was like what was left after the "tide went out and the market found out who was swimming naked".

Certainly. The 2.4 kernel and IBM embracing Linux at around that time pretty much made all proprietary Unix legacy.

The desktop took longer with less well-defined transition points and, arguably, MacOS with its BSD foundations (and command line option) ended up being a good alternative for a lot of the non-Windows crowd--though Windows is still dominant as a desktop/laptop OS. (Windows/Azure are, of course, still major in backend corporate environments as well.)