If they didn't want to live in Ukraine, they could move to their fatherland - Russia.
We should be rather careful with such arguments, because they are very easily dismissed with "if Albanians in Kosovo didn't want to live in Serbia, they could have moved to their fatherland - Albania". But they didn't. Serbia was bombed and on Serbian lands ethnically Albanian state of Kosovo was created, with full support of the West.
We can be either
imperialist i.e. we think that countries should be able to keep lands and regions of different ethnicities under their control. In such case we should support: Serbia controlling Kosovo, Russia controlling Chechenya and Ukraine controlling Crimea and Donbass.
Of course, we can also be
against imperialism; in such case we should support the rights of ethnic Albanians in Kosovo, Chechenyans' right to their independent country and ethnic Russians' struggle in Crimea and Donbass.
However, something strange seems to happen here - people
on both sides (the Western camp and the Russian camp) take staunch imperialist position when it suits them and then with happy smiles on their faces argue vehemently for an anti-imperialist position, when it suits them.
Of course, it isn't strange at all when we realise that we live in a world in which superpowers fight for influence and resources, and in this power struggle they will support any positions, no matter if mutually exclusive, to justify their actions. Welcome to the real world of adult people, children. That's why, instead of trying to claim moral high ground, we should admit that it's all about strategic interests of our respective countries/alliances. We, Poles, support Ukraine because we think that it suits our long-term strategic interests to keep Ukraine stronger and Russia weaker. As long as we have Putin and his mafia in power in Russia, and as long as people like Dugin are listened to by Russian ruling class instead of being locked up in a lunatic asylum,
our position seems totally valid from Polish interests point of view. If, on the other hand, we had democratic Russia with, for example, Navalny as president and all the Dugins safely locked in психу́шки, would we be so eager to support Ukraine, with their UPA cult, official Bandera celebration year, and lovely Shukhevych poststamps?