They can’t really update any of the iPads without updating the Pros first though. Think about it - the Pros are still sporting an A12 based chip from 2018 (currently marketed at the A12Z, but it’s still the exact same chip that shipped with the 2018 Pros, just with an extra GPU core unlocked). The Air also has an A12 chip (the less powerful version from the iPhone) but upgrading it to an A13 or A14 would make the Pros look old and inferior. The iPad is still using an A10 and I could easily see it jumping up to an A12, but not before the Air gets updated to something better than the A12... therefore in my opinion updating the Pros needs to be the first step in refreshing the iPad lineup.
The iPad mini received A12 the same time as iPad Air. I don't see any reason why the $329 can't get A12.
The Air still has a better display (P3), better Pencil support (laminated), more base storage, and is physically thinner. Think of the $329 iPad as iPhone XR while iPad Air as iPhone XS.