In the aftermath of WWII, the nations of Europe were economically devastated. The Eastwrn European nations were occupied by the Red Army, so not much could be done for them. The Western European nations needed assistance lest they too fall to Communism. Hence the Marshall plan to economically rebuild them and NATO to protect them militarily.
Once they got back on their feet, though, by about the mid-late 1950’s or so, they returned to prosperity. This was not reflected, however, in the defense budgets of these nations. For the most part, they were quite happy to take the free ride offered by the US defense umbrella, and thereby use their wealth to promote a socialist domestic agenda.
Now that the Cold War is history, Trump is absolutely right, NATO is obsolete. It is high time these prosperous nations provide for their own defense. Should aggression from Russia or some other source threaten Europe, it may still be wise for the US to assist in defense of Europe, much as we did in the first half of the 20th century. Such decisions should be made based on the actual geopolitical situation, though, and any such alliance should not be an excuse for European nations to neglect their own defense.
Your ass is "over". You know your guy Putin is in the process of restoring the Soviet Union, or something damn close to it. Only this time it would include the oil/gas-rich Middle East. And, as I posted above, Trump said the alliance was still very important to him. Just that the deadbeats need to set up to the plate and do, or at least pay, their fair share.