The Russians do not completely have to mobilize their entire army into the Ukraine. Use Spetnaz, gain complete air dominance, and eastern Ukraine is theirs. All could be done in 24 hours.
I don't disagree, the Eastern Ukraine is basically theirs for the taking if they want to pursue that option.
While they could do it, there are a lot of downsides. Putin isn't really holding the strong hand he projects. It only looks that way because the US has an idiot for President and the West in general is weak.