...
NATO is now nothing like it was formed and the U.S. should reconsider if it wants to remain. Given the current NATO obligations, the U.S. would be required to militarily support Tajikistan if they had a border conflict with Afghanistan because Tajikistan is a member of NATO and Afghanistan isn't.
By withdrawing from NATO, the U.S. could decide to help some country if we saw just reason but would not be obligated, under treaty, to do so
I think that you are wildly underestimating the ability of the EU, which is not a military alliance, to resist Russian expansionism, especially if they use the "divide and conquer" strategy that has worked so well since the time of Julius Caesar. Once Ukraine is absorbed, Moldova would probably also go, and pressure would be put on all of the former East European bloc to fall back in line with Russia. But for NATO membership, of course. And that's the point.
BTW, Tajikistan is not a member of NATO, and there would be no treaty obligation for the US to defend it if it were attacked by Afghanistan or anyone else. It is a member of the Russian-led CSTO, so Russia would be more likely to intervene.