Hot swappable motherboards are not that necessary...you have redundancy for that. Besides, the default motherboards you buy from vendors is not hot swappable anyway, because of the power management.
So special motherboards, special power supply, special (rack) case...you likely see already that it is cheaper to run a second system, hardware wise. Let alone the software and maintenance fees...
Solaris is nice, but you do not see me shed any tears if it would go the way of the dodo.