It's not just about the next 4 years, we can't base our defense on a country we can only rely on for 4 years at a time.
There were already problems with Bush Jr. then Trump once, and now Trump again.
You'd have to be insane to believe this is just a one or two off, and things will get back to normal.
Normalcy regarding NATO ended already with Bush.

The times where USA was the deciding factor in most world policies, because USA was backed up by the majority of the global economy is over.
USA loses just as much as Europe and many other US allies. Trump said our so called friends are our true enemies. Trump believes Canada, UK, EU etc are enemies, and he has said directly that Russia and China are "not so bad". Despite Russia is invading in Europe, and threatening nuclear war, and China is behaving ever more aggressively in the China sea and against Taiwan, and in general against their neighbors.