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.