They've done it once. They could very easily do it again.
What you're arguing is that might makes right. Actually, it doesn't.
These people in Latvia didn't ask to be there; they were born of Russians forcibly relocated there by Stalin, or the descendants of same. They didn't ask for their lot in life. Punishing them for the sins of their grandfathers' and great-grandfathers' rulers fits no definition of justice that I can comprehend.
If you think that denying them citizenship based solely on their ethnicity is a good thing, then you're arguing that rights are, in the end, completely alienable and subject to the whim of the majority.
I'd love to hear you argue that vision with the same intensity when you're in the minority.
You don't seem to want to acknowledge that the only one denying them citizenship is themselves, precisely because they refuse to take a test in Latvian language and history. That proves they don't consider themselves Latvians first. Like you they are offended at the idea that Stalin had no right to make them Latvian citizens, so they refuse to obey the law. If they choose Russian nationalist pride over law and order, then Latvia is better off without them.