There is 0% chance that NATO intervenes military, even without the fact that Putin basically told the world that any country that intervenes will be nuked.

Regarding economic sanctions: yes Russia will suffer from them, but the thing is that it is impossible to know how high Putin values the prospect of dissolving Ukraine into Russia, so it's pointless to try to reason about it (not that I'm saying you shouldn't).

Putin is 70 years old. He might just feel like he hasn't done enough with his life and decides to go all in, who knows?