Spitfire,I think Ukraine would be forced to negotiate a peace deal with Russia and the war would end, at least for now. They would have to give up Crimea and the Donbass and commit to disarm and not pursue NATO membership anymore. Not necessarily a bad thing in my view, as it would mean less death and destruction all around. Ukraine isn't really significant in the big picture. Certainly it is not as strategically important as Taiwan is, at least from the US perspective.
As far as NATO, it wouldn't change a thing in terms of NATO security. If Russia attacks a NATO country like Poland then WW III would begin plain and simple. So it is not likely Russia is going to instigate WW III. But say Poland attacks Russia on their own, NATO wouldn't be obligated to help. So hard to say for sure but I think NATO will press on with business as usual and continue to build up within it's territory.
Doesn't really matter though because Biden has been told to fight Russia down to the last Uke. And even with all the goodies and money that Uncle Sam is sending, it won't change the fate of Ukraine significantly IMHO. Once the main Ukrainian army is defeated in the East then Kiev will be forced to resume negotiations less they want to just loose more territory. Personally I think the only way the Ukes could win the war is if NATO countries joined the fight, but that is not likely to happen because it would mean WW III.
Taiwan has strategic importance from the US perspective ?
Significant symbolic importance is the big thing now.