There seem to also be natural gas reserves in the sea west of Crimea.
If Ukrain joins NATO Russia would not be able to hold onto Crimea and
could become a Petrol state.
With renewables replacing part of the Gas energy demand long term (10/15y) this
could mean that Ukrain could replace all Gas imports previously done from
Russia. At the same time a modern and well-working Ukrain could influence Russia
(as a unintended side-effect) in a destabilizing way (anti-authoritarian Government movements).
So if my info is correct a stable NATO Ukrain could, in a decade or so, lead to a situation where Russia is making very little money, no longer being able to afford
much military besides for self-defense usage. That there is also a shrinking populating constantly reducing the amount of "potential soldiers" isn't helping. Neither is helping that a lot of middle tier and upper middle tier Russians have left Russia in the last many years (I think, not 100% sure), this reduces the chance for economical growth through new businesses.
If all of this is true (I'm not sure) then Putin might believe that they have to either take over or majorly destabilize Ukrain if Russia like it's currently is want to continue to exists. (This doesn't mean it would disappear, but it would need to change likely at the cost of the current oligarchs.)