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.
It all comes back to SPARC. Price to performance. If Sun had amazing high quality x86 servers and workstation by 1998, their software would have been perfectly fine. Its not like there was anything missing from Solaris at that time.
Many people were not unhappy, they just realized that they could get a faster cheaper machine, and run the same software on a linux. Even if linux was less optimized, still end up cheaper overall for the majority of workloads.
Its not like running linux on unsupported PCs in the early 2000 was that much fun. But Sun by 2002 they were still trying to kill x86 based stuff. They bought x64 server line in 2004.