You need to look up what the alliance is for.
Notably, I believe NATO goes much further than simple defense vs. Russia. Yes, that defense was an original objective, but, more deeply, if Russia were to just vanish or turn into Winnie the Pooh, you STILL have the problem that a Europe with pieces corkscrewing off on their own is incredibly dangerous. As I mentioned above, we’ve seen that playbook before, and now you can add nukes.
So... a very “tight” commitment and structure, reflective of a “one for all and all for one” philosophy, is necessary, with a big dog (the USA) as a glue to help hold it all together and knock heads to some degree, if need be. Additionally, few of NATO’s members are “can do it all” militaries, so, they have to lean on each other. It is the only answer...
That said, all the major Euro NATO countries are now up to 2% of GDP defense spending, except Italy*, and NATO outside the US is as a whole up to that 2% figure.
*Maybe Giorgia Meloni is cozying up to Elon for a reason — hahaha.