Anyone who thinks the war will be over when Ukraine falls is a gullible idiot. The Ruskis will march straight to Moldova after the takeover of Ukraine.
Every non-NATO state on the border will be targeted and incorporated either directly or by being turned into a loyal authoritarian state a la Belarus.
And if the US leaves NATO, that undermines confidence in NATO. The alliance will fall. At that point, either the EU will have to step in and supplant it -- in which case the US will no longer be even be PART of the mightiest diplomatic alliance in the world, much less influential in it -- or else there's no reason to think Russian aggression will stop endlessly escalating until all-out war between nuclear powers is on the table.
Additionally, there is already a lot of discussion about nuclear proliferation.
Some nations did not develop their own nuclear weapons because the US guaranteed for their security in exchange.