Sorry, I didn't mean to imply there are no minorities in Crimea, or that they shouldn't be represented. All nations have minorities, and all democratic governments need provisions to protect them from oppression. What I meant was that, as you say, "its population is mainly Russian," which is disparate from the rest of Ukraine. Perhaps I should have said, "Geographically distributed demographics ought to be governed separately."
I see what you're saying, but it's too much of a clusterfuck for any sort of sane solution. Many Russians don't see post-Soviet countries as being separate from Russia. There's a similar part of the population that has no desire to assimilate into local populations. What ends up happening is that there are many Russian enclaves all across eastern Europe that want a Russian-friendly home. This is Crimea, Southern and Eastern Ukraine and other post-Soviet countries. As a consequence there is a Russian cultural empire growing in non-Russian countries. So letting the demographics decide the governing body is how the Russian empire spreads across all of eastern Europe and the Middle East. Not great. But as I said, there is no sane solution. My solution is to throw my hands up in the air and hope there won't be any bloodshed.