The real benefit of NATO is to force European wealthy nations into having skin in the game.
Let's face it...no matter what might happen, the US is carrying most of the water. Everyone knows it, the US knows it. However, putting real skin in the game from UK, France, Germany, etc in theory keeps them aligned with US interests, even if the US is going to carry the load.
The US has every right to have been critical of NATO in recent years, but it's not as simplistic as "we're paying the most, they need to pay their fair share." The issue is much more what some NATO countries have been doing empowering Russia and China.
The entire situation with Germany and Russia is a perfect example of the point. Theoretically NATO countries should not be empowering and enriching and appeasing a bad actor like Russia. But Germany has been double dipping, relying on the US for defense if shit ever hit the fan, and empowering Russia for the sake of gas. That should be a no go, and that's where the US is right to be tougher with NATO than we have been. No other country is going to have skin in the game like the US in terms of live and treasure, it's not going to happen, but they SHOULD have skin in the game other ways, like producing their own energy to isolate threat states.
I'm hopeful that this Ukraine adventure has been a come to Jesus moment as far as this goes, and the consequences of not being aligned on the posture toward evil regimes.
As far as who should be allowed to join...it needs to be looked at through that lens. In theory, I'm come one come all, but I think we need to very carefully evaluate whether they can be counted on. For example, I'm all for Ukraine in this war, but given the history there, the Russian influence, etc, to me before the war they would have been very far away from being an acceptable add. There is a reason why Putin thought the Russians would be welcomed with bread and flowers. You can't add anyone that is less than 100% committed to the ideals of Western-style democracy and alignment with the West. You can't add anyone that you worry about undermining you.
Ironically, I'm much more open to adding Ukraine (not going to happen) after this war than prior. They've proven where they stand vis a vis Russian, with blood.