If those Eastern European states didn't join NATO, they would have ended up dysfunctional kleptocratic autocracies that were effectively client states of Russia, like Belarus. And thats the best case. The worst case you would have had repeats of what happened in Chechnya. The whole notion they could have remained "neutral" was a pipe dream
Russia had a chance to reform and join the West but they chose the path of rebuilding regional hegemony
Also I would note, the military spending of NATO, at least in Europe, nosedived after the Cold War ended. After 9/11 there was a bit of a bump, but not much. So how does that align with the notion of them being some threat to Russia?