If Biden intended to conquer Ukraine, why did he use minimal force? Do you really think Russia went “all in” against Ukraine and hasn’t steamrolled Zelensky?
Even if it were a simple war of attrition, all Ukraine has to do is kill 5 Russians for every one of theirs and they win. Given that most of the Russian army appears to be little more than an armed mob, that isn't impossible.
Look at it in detail: Russia has a large air force on paper. How many are actually operational? How many trained pilots does he have? How many trained air crews? How many bases are within operational range given the fact that that fully loaded aircraft must fly low and slow to avoid Ukrainian air defense? Given all these constraints, what ops tempo can they achieve? Not much.