Except you predicate all this on assumption that Ukraine would join NATO, which probably wouldn't happen in any predictable future. There is no chance Germany, Turkey and Hungary would all vote for it if the Russia was opposed. Hell, if Putin didn't seize Crimea, it's likely that Ukraine would voluntarily stay in the Russian sphere of influence.
Ukraine was about 50-50 pro West and pro Russia, before 2013 by seizing Crimea and instigating revolt in Donbas, Putin actually disturbed this balance as he effectively removed pro-russian population from Ukraine and made the rest very anti-russia.
Also by seizing Crimea, Putin effectively prevented Ukraine from joining EU or NATO, unless they cede it. NATO doesn't accept countries with territory disputes and I doubt EU would accept one of the more corrupt countries with such an issue.
Basically, Putin already achieved his goal of preventing Ukraine to join western organizations in 2014. However, Putin doesn't like to compromise, he wants to win. He is a maximalist and he sensed the West was weak and Ukraine was weak and he could have it all, instead of just Crimea.
Also, it baffles me how Putin couldn't have calculated that his actions in 2013 and this invasion would push Sweden and Finland to NATO, effectively increasing his borders with the alliance. His actions smell of stupidity, arrogance and pride. I'm sure that in his mind Ukraine must be a part of Russia whether as a province or semi-independent country in the style of Belarus, he doesn't care, as long as they listen to him. Everything else is just propaganda and excuses for the gullible.