WI: Russia Does Not Expand East?

Let's say for whatever reason no Russian polities expand eastwards. What peoples would live in places now dominated by ethnic Russians?
 
I would expect China and Mongolia to move into Siberia (or for Siberia to be its own nation) while Japan moves and takes Kamchacka as a colonial province
 
Why don't they. Russia has every reason to expand it's frontiers barring extreme degrees of depopulation.
 
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