What I've seen look like war crimes, what is the argument for qualifying them as genocide?

Stalin's (and bolshevik's in general) crimes are horrible, but ethnic Russians weren't safe from them either. Have you seen breakdown of their victims by ethnicity?

The point is that Russia is the largest country by land area, composed of a great many regions and ethnicities, and yet all the language-speakers have vanished.

This didn't happen by accident. It is the result of a program of systematic, violent, erasure.

"yet all the language-speakers have vanished"

Now that is just false. [0][1]

[0] https://en.wikipedia.org/wiki/Languages_of_Russia

[1] https://en.wikipedia.org/wiki/List_of_languages_of_Russia

The most common non-Russian language is spoken by just 3% of the population.

All others are below 1%.

Which is order of magnitude higher than, for example, in Canada, where less than 1% in total speak indigenous languages. [0]

Or look at Poland where only 1.8% in total speak a language other than Polish. [1]

[0] https://en.wikipedia.org/wiki/Languages_of_Canada#Indigenous... [1] https://en.wikipedia.org/wiki/Languages_of_Poland

The lengths this guy will go to, to deny his country's ethno-fascist roots. Russian-exceptionalism is a hard drug to come down from.