If US pulls back from NATO, and Europe builds up military power to compensate, then the US loses this de facto leadership seat of an empire.
Today, the US appears in parallel to be doing two things:
1. Causing fragmentation in Europe, by promoting right-wing nationalist politics in the EU
2. Threatening to drastically reduce their role in NATO
At the very least we can both agree that these two efforts are completely in contradiction with each other, and it's very unlikely that Europeans will want to go for more fragmentation without the military power of the US on their side, right?