That would be entirely up to the maintainers of Mercurial if they would distribute pre-built packages for Solaris 10. If you're concerned, you'd be better off making the case to them to do just that.
It's about the concept, not about Mercurial per se: nobody should be forced to have to bootstrap and port an entire programming language just to be able to use a newer version of an application. That's insane and irresponsible, not to mention selfish.
How strongly are you claiming "nobody" here? Does the cost of supporting niche platforms ever get too high in your mind? If so, what is an appropriate line to draw for cutting off explicit support?
What you're saying is that if I had to upgrade to a newer version of Mercurial which used software written in Rust, I'd have to bootstrap and port an entire programming language, just to use a newer version of my revision control software?