That Oracle is moving to Linux on Sparc tells you they still think there is an enterprise business that Exadata/x86 couldn't win (after years of destroying Solaris). IBM+Sun would have been most of the E-biz and those customers would have been more comfortable with Oracle as just the DB provider.
Solaris on PPC existed at various times, IBM could have provided a convergence path for the Fortune 500, AIX/Solaris/Linux/Java all on PPC.
It was actually a close thing, but someone else will have to write that story.
SPARC died the moment every developer had a good enough x86 desktop running a Unix flavor (and Sun decided it'd be happy not to build desktops anymore). At that point, we cease to imagine deploying our stuff in SPARC. We test on x86 and make it run well on x86 before it gets compiled for SPARC.
At that point, unless the SPARC hardware has some definitive cost/performance advantage, we'll buy x86. SPARC is for legacy.
Same applies to POWER, BTW. How many new apps have you seen in the past 10 years that were designed for POWER?
What accelerated it even further was when Linux went 64 bit and hardware improved to the point where it was a suitable replacement for big iron Solaris (the SunFire 6900 for example). That was around 2005 from memory.
And the 6900's features with regard to HA in the field fell well short of advertised.
Now you can get a Linux box with 4Tb of RAM so no-one should buy Solaris over Linux.
Sparc is also effectively dead. Solaris and Sparc exist as ongoing projects only to the extent Oracle is contractually required to maintain them--e.g. to Fujitsu. If you accept that Solaris is dead, then by the same evidence Sparc is dead. If you believe Sparc is still viable, then by the same evidence Solaris is still viable.
Solaris on PPC existed at various times, IBM could have provided a convergence path for the Fortune 500, AIX/Solaris/Linux/Java all on PPC.
It was actually a close thing, but someone else will have to write that story.