But Sun in the 80'ties used 286 and 386, IIRC. It was not until mid-late 80'ties the first SPARC processor was introduced.
I don't know whether UltraSPARC-II, III or whatever is the most state of the art is that superior to other makes. I suppose it depends on what you use it for and the architecture. For a desktop computer, I was not overly amazed by the Ultra-10 (yet the 440 MHz version) performance over a top class PC at that time, in particular if you took in consideration you could get three top class PCs for the cost of one Ultra-10. I suppose the desktop systems more and more became IBM PC compatibles with SPARC instead of X86. As a server, they at least used to be excellent due to great bus capacity. In a blade server or even bigger application, I can see if the overall capacity makes it an attractive platform compared to Itanium, PPC and whatever other major architectures there are.
AMD also has a RISC past with the 29000 series, which they dropped in 1995 in favour of making profiable Intel clones as we all know. I suppose at least a bit of the spirit remains with the development of Opteron and future installments. I don't think Sun would be hampered by joint developement with AMD if that was the future. It could still be a SPARC, but maybe with some new concepts and manufacturing shortcuts. Hey, maybe it would even cut the price a bit, so they could afford to put the same architecture in all computers again?