I’m not sure if you meant that it was denied or just that it hasn’t become a member (yet). First one isn’t true but second one is. The process is ongoing and they’ll fairly likely become a member at some point, like mentioned.
I just do not see sufficient evidence that Finland was under a threat that was only deterred by US military spending.
We were close to NATO for a long time but felt that actual war in Europe and Russia attacking was too unlikely for us to actually join. Russia had to come and dispel that fantasy.
NATO isn’t supposed to be the only thing preventing it but it sure does bring us security that it won’t happen and if it in some unthinkable scenario did, we wouldn’t be left alone to fight it.
Looking at Russia and how they’ve generally treated their neighbors, saying that NATO has saved Eastern Europeans from trouble is a fairly believable argument IMO. Just look at those who didn’t join and what has happened with them. But of course it’s what ifs.