First, compare what the US has and brings compared to the other 31 nations in NATO.
From last year.
We are the main deterrent in NATO. We are also the overwhelming glue that holds NATO together. Which is why Sec General Rutte was chastising the EU last week. The EU member states would likely fracture should the US leave NATO and withdraw our forces.
Now, this is from last year and some things have changed since then, notably Iran and Venezuela but it speculates that both NATO and the US would have some consequences and not all good for us nor them. Again, all speculative but very plausible.