The nature of war fighting, defence and offence has changed dramatically since the fall of the Berlin Wall, the withdrawal of the Soviet Military from Eastern Europe and the fall of the "puppet" communist regimes in Eastern Europe.
Then (1960s, 70s and 80s) we thought,. and were prepared to fight, in terms of massed armies ranged across Europe, from the Alps to the Baltic. Largely armoured, you had the NATO corps, each of up to 4 divisions , German, US, Belgian, British, Dutch and German again. The second wave was a massive airlift of American troops from the continental USA , practiced in various degrees in the big NATO exercises - which almost always included a "Reforger" component from the USA. The armies were manned, with the exception of the British and latterly the US, largely by conscription. The opposition, Group of Soviet Forces Germany was deployed similarly, albeit in a more offensive posture, backed up by their Warsaw Pact allies and second wars drawn on mobilisation from the Soviet Union.
The whole business has changed. Formations are smaller, more agile and far better equipped, the brigade has replaced the Division and Corps as the fighting formation. Brigades are far more practiced, trained and expect to fight alongside brigades from other nationalities; in fact multinational brigades are common. Conscription is much less of a feature. Equipment commonality is much greater, communications, command and control more effective and faster; artillery and anti tank guided weapons are far more effective, particularly in defence, and the development of drones both for surveillance and delivery of munitions has opened up a whole new dimension.
All this, training and equipment, has become much more expensive of course, but you now have a vastly different battlefield, to that which NATO was created to fight on.
There is a need to spend more, there will always be a need to spend more, however the core reliance upon the US for reinforcement in manpower and weaponry is much reduced - "Reforger" has not been played for decades, and US withdrawal from or prevarication over NATO will have less impact than it would have done in the days of the Cold War. Europe would lose the US nuclear umbrella, but the UK and French strategic deterrence would remain.
My point is, NATO has changed, Europe has changed, militaries have changed. US withdrawal will be a great blow, but not as fatal as it would have been to "the old order". New countries are emerging as drivers within NATO, in particular Poland and the Scandinavians. The current regime in the USA has cast significant doubt as to wether they could actually be trusted to follow the core rule of the alliance, article 5 of the treaty, an attack on one is an attack on all. NATO and Europe will have, are having, to learn to live without the USA.