I am a long-term SunOS/Solaris user (~20 years) and what killed Solaris is they neglected Solx86 for years for fear of cannibalizing SPARC sales. What happened was devs got Linux boxes to replace SPARCstations (using them as cheap X servers) then x86 and x64 got good enough for servers, and everyone ditched Solaris in order to get onto the commodity hardware (Linux wasn't really a factor in this decision, it just happened to be the dominant Unix on that hardware at the time, could easily have been FreeBSD). Thanks to Microsoft and their NT pretensions, supporting hardware vendors (e.g. storage) were manufacturing compatible kit too. It didn't help that Sun managed to forget that they were a business, and a hardware business at that. Giving stuff away to drive sales is a proven strategy, but no-one was buying SPARCs. Turns out it was cheaper to buy PCs and accept failures and workaround them, than to buy a "real" server that wouldn't fail and if it did, you could hot-swap anything. Call it the Google method.

Sun could have had a compelling desktop-to-datacentre story, all Solaris, on Intel PCs all the way to SPARC near-mainframes. But they succumbed to short term thinking and killed the goose that laid the golden eggs. The alternative to the Oracle acquisition was for them to go the way of SGI...