If the current state if the Ukraine war is success from Russian perspective how would a failure look like?
Rational people still, in 2026, operate under a notion that Putin has some definitive target. Well, he does not, the invasion IS a target. The process is a target. So while the war goes on, the whole operation is a success to him personally.
Same with hypothetical NATO invasion. It's target won't be neither victory over the alliance, nor any gains in territory or cities. The point would be to create a permanent ongoing invasion(s) inside NATO member's borders and sustain it.
As it is it's kind of a mess with many dead on both sides.