I'm still not sure I buy into this. I could see Apple doing basic updates the 11" and 12" iPad Pro (A14X, 5G, etc) and then slotting in a higher-tier 12.9" with new display technology similar to how there are multiple 13" MacBook Pros.
One reason I don't see the 11" being completely removed is that Apple would then have a large hole in their lineup at this size that they did not previously have as the iPad Air is not able to fit all of the use cases of this size class in the past. The main factor I'm thinking of here is storage. While ProMotion, better speakers, LiDAR, etc. could all be waived off as "Pro features," the iPad Air only goes to 256GB of storage and I don't see Apple making people go to an entirely new size class for 512GB storage which has already been offered in the 10"-11" size class for years. That plus the fact that Apple very intentionally chose 10.9" for the iPad Air. Why not use economies of scale and use 11" unless you were saving for future differentiation.
It's not like with MacBook Air only getting 2TB as the MacBook Air has never had more than that, but to tell all the mid-sized market that they can no longer get higher storage seems odd to me. Plus it seems like odd money to leave on the table for the customer base that would pay the up-charge from the 10.9" Air to the 11" Pro.