The last Sun systems I operated -- x2200s not up to Sun standards -- ran GNU/Linux on Opteron in an HPC cluster.

I would say that there were three factors that superseded Sun, along with most of the other proprietary workstations and servers: FOSS Unix, commodity PC-compatible hardware, and TCP/IP.

The big vendors used to be differentiated on how they communicated on a network, shared files, type of bus architecture, and workloads, and so each company or unit would choose a vendor and rely on them for solutions, and the staff had specialized expertise in those systems.

But all those vendors found themselves adopting TCP/IP and Ethernet. And Sun's NFS was widely adopted. X11 also became standard. The BSDs already had a wide hardware compatibility list, so that spare machine with no OS license could be returned to service. A few generations of college grads had direct experience with Unix and building PCs. Once Linux on PC-compatibles began showing up in the server room, Windows NT was mature, and software vendors were porting to Linux, it was a fait accompli.