I sort of agree, at least as far as Crimea is concerned. It was never really part of Ukraine proper anyhow was it? Was sort of autonomous republic.
With the other southers and eastern provinces it gets a lot more hairy, and a breakup could set an awful presedence, which is why the baltic NATO members invoked article 4.
And if the Russians threaten Western Ukraine, NATO won’t be able to hold back the Poles...It was their former territory.