At a minimum, assuming they do ultimately take the country, they will have to conduct a long term and costly occupation to support whatever puppet government they install... because the moment they leave, that government is at risk of falling.
I really don't get why this was such a hard thing to grasp ahead of this invasion. What percentage speak Russian, or even whether they have voted for Pro-Russian candidates in the past, isn't necessarily synonymous with the percentage who would support their country being invaded by Russia. Or be supportive of a Russian puppet leadership.
Putin clearly got high off his own supply in this regard. But a lot of outside observers did as well.