
The problems started before Schwartz
Several years before McNealy stepped down, Sun management put the "Solaris on x86 processors" project on hold. No new development was done for a couple of years. IIRC this was around 2001-2003.
During that time, clock speeds on x86 processors hit a sharp growth knee and rose from about 1.0 GHz to 2.8 and even 3 GHz. Sparc cpus stayed around 1.2-1.6 GHz. The prices of x86 servers went lower, while Sparc servers stayed about the same. Ordinary bugetary economy forced the start-up segment of the market to buy x86 hardware rather than Sparc. The lack of Solaris availability on x86 forced them to install Linux. That began a shift in the market away from Solaris to Linux.
Sun management revived the Solaris x86 project around 2003 or 2004 and made big committments to x86 (both 32-bit and 64-bit). They've also tried hard to make Sparc competitive with x86, but they haven't turned back the tide. It will take many more years of effort to regain what was lost in that one bad decision.