"Why should the U.S. continue to have access to these bases, or receive support from allies’ naval assets, air forces, or even intelligence services, if it tries to take sovereign territory from a NATO member like Denmark? "
https://www.politico.eu/article/donald-trump-europe-greenlan...
That's literally what they are. American forces appeared in Germany in 1945.
De facto and de jure are two very, very different things...
(not saying the US forces are occupying Germany, just commenting on op's logic)
And there is a kernel of truth in it. The USA likely wouldn't give up Ramstein under any circumstances safe the German military mobilizing against them, the base is (was?) too important for the US. When Trump invades Greenland we will see this play out (how the base stays active and Germany is powerless to stop that).
So if the US decides to resign from NATO, they would likely face challenges directly with Germany regarding their existing agreement.