Their complete systems were competitive in terms of having a good (price, features, usability, reliability, support, ...) tuple, but that doesn't mean every attribute of their systems was the best of all systems on the market. I know people who bought Solaris systems, and nobody bought it because of the CPU.

You say they were the market share leader through the "early 2000's". In 2005, their OS was open-sourced. Sure enough, everyone I knew using Solaris since then has run it on x86. People like the OS but only used SPARC when they had no choice.