I think the type of people who buy the foldable are likely to be very different buyers than those buying the Air. (Saying this as someone who loves my Air and would be definitely interested in a foldable depending on specs, but think I am the exception, not the rule). I suspect the foldable will be aimed/marketed at "power users" and the Air most certainly isn't. Lots of people buy the ProMax not because they necessarily need the features but because "it's the best" and I suspect a foldable "iPhone Ultra" will attract a significant portion of those buyers.
I know everyone thinks the Air is the test run for the fold, and I am sure they will take some of what they learned from the Air when making the foldable, but my hot take isn't that it's it's the test run for the foldable, but it's a test run for the Air form factor becoming the "standard" phone (like the MacBook Air is the "standard" MacBook). They're going to add a second speaker, and an ultra wide camera, do further battery optimization, and release it with the Pros/Foldable as the "standard" model, with the "base" model coming out in the spring (as currently rumored) replacing the "e" line. That allows Apple to effectively raise prices without raising prices (and I'd argue get an objectively better phone as the "standard" model in the fall - although I suspect people here will disagree with that)