Trump pulling America out of NATO won't destroy the agreements between all the other countries involved. Americans do like to think that the US military forces are the only significant part of NATO alliance, but that's just the usual American ignorance about the rest of the world.
The European union has a larger population than the US and a larger economy. Russia couldn't pick of EU member states one-by-one - it's been picking at the Ukraine for a few years now, and France, Germany and the UK are each quite a bit bigger and richer than the Ukraine.