That's precisely why I expect NATO is going to fall apart in the end. It's going to be really hard to justify austerity to ramp up military spending when there's no credible threat.

US threatens anyone who will leave with massive economic sanctions. And since all the leaders are cowards who refuse to band together the US has a far stronger negotiating position and always wins.

but those countries do not face a credible military threat from anyone

If they leave NATO they will face a threat from NATO, duh. Remember, war is a racket, and NATO is a racket too.

I do not know if the same "logic" holds true for others, especially the larger nations, but to many in my nation it's "if we leave there's no telling who of the neighbors is going to absorb us". Perhaps going by the "there's always a bigger fish" there's a feeling that nations inside of the banner at least on paper are obliged to not attack each other in an effort to deter the larger opposition.

I personally though this is a bit of a fantasy, as Greece - Türkiye showcases that situation is far from ideal and wouldn't it make more sense to try to reason and integrate with the larger (outside) nation? As otherwise you're really leaning into a narrative that the "outside" is somehow born evil, which should feel quite hollow to anyone hearing it.