He probably sees Europe as too meek to do anything more dramatic/substantial. And believes that without NATO, Europe would buy more US weapons that they now get "for free".
[1] https://www.dirittoue.info/u-s-legislation-restricts-preside...
After NATO 1.0 is declared dead and burried, Trump might as well backpedal and start negotiating NATO 2.0. Which would be light on US military commitments and heavy on European arm purchase commitments. And he seems to believe (not unjustifiably - see Nord Stream sabotage) that the European leaders are spineless enough to accept a NATO 2.0 deal.
This will not be unlike Trump's thinking: "I'll build a wall and the Mexicans will pay for it".
Wild theory, yes. But we live in wild times, unfortunately.
Buying weapons from an unreliable and possibly adversarial (former) partner would create strategic dependence and weaken Europe’s defense autonomy. => It would be stupid.