The EU can pick up and work out a new security arrangement for their neighborhood.
But we can't leave NATO right now even if that is what we wanted to do. Obviously, Trump can withdraw but he's not going to get support from the Senate.
Actually I said this!
“...This new arrangement may or may not require the US to be a full time member. ...”
The US can be involved or not. It should depend on the situation and conditions during the formation of the new European security arrangement. Again our involvement could be as a full member or as an “interested observing member” like Sweden was with NATO for years. One of the biggest reasons I think for refusing full membership is whether the Europeans particularly the Germans (very particularly the Germans!), & others (UK & France) take security as a necessary priority. The US should no longer subsidize them! The US would only be a full member if those countries take European defense seriously!