If that were true, most illumos users today would be SPARC users. There would be more than a couple of people working on SPARC support, and not merely as a part-time hobby. There would be software support for a SPARC machine that was sold some time after 2011.

Instead, something like 99% of the people running illumos are doing so on 64-bit x86 machines. Dropping SPARC support will allow us to move forward much more easily with enhancements to the dramatically more relevant x86 bits. If anything I expect it will allow us to do interesting things that would garner new interest, like using Rust to implement bits of the operating system.