Obviously not since the agreement amongst existing members has to be unanimous.

"Valid" as in feasible? No. The war was started to keep Ukraine out of NATO, and Ukraine wasn't fully backed by NATO in the war because it isn't willing to go to war with Russia just for Ukraine.

The whole war was started because the Russians consider Ukraine in nato to be their red line, and the war was lost by Ukraine because the NATO leadership never seriously considered putting Ukraine in NATO if that meant direct war with Russia.

So no, it isn't a realistic goal at all. The war ends only when either

1. The Ukrainian state collapses
2. A deal is made in which Ukraine does not join NATO
3. Ukraine pushes back Russian forces so much that they collapse the Russian government.

Scenario 2 seems to be the most likely outcome.

I'm not an expert, but have read a decent amount on this. Others may have more and better info.

With that said, even if an Article 5 invocation won't bring the US into your fight, it provides a hefty infrastructure of value to countries in it. From basing, to logistics, to intelligence, to aid, it is valuable. Now the politics of it are complicated and the US can hinder some of that value, but it still means that in Europe if Russia provides an Article 5 reason, other countries in NATO can choose to help in various forms. That's not nothing. It's also faster and less arduous then negotiating individual defense treaties with neighbors and others.

So yes, overall probably still worth it. Even if just as an entree into other alliances.