Maybe the US is adopting the position it is now because it is convenient for its own ends.
Charity comes at a price. If you are willing to outsource your defence to some other party then there are risks associated with that if and when the charity stops/the status quo ends.
Maybe if the NATO countries had responded during Trumps first term to his requests on more defence spending we wouldn't be in the position we are now.
US help has always come at a price and has always been offered somewhat reluctantly and many would argue rightly so. See WW2 for an example.
Europe has had the option (and warning) to step up and has failed to do so. Now it has very little choice in the matter.