I don't think it is either or, NATO is just a symbol for western alignment which reaches the same ends you describe. Beyond whatever foundational context, the three conflicts related the Ukraine and Georgia were precipitated by this (specifically most recently with the Brussels summit in 2021) which suggests it is a very definitive line in the sand, regardless of whether or not it is the actual reasoning. This is the third time the west has tested that line, it is the third time Russia has responded in this way, and it will most likely be the third time the West does nothing but implement sanctions because it isn't as important to them as it is to Russia.

This isn't a justification for actions, it is about understanding the situation through the chaos.