I agree with OP on AVP and expected it to fail too, based purely on cost, but the foldable iPhone will be different. Apple tried to pioneer Spatial Computing and start an entirely new segment with AVP, and history is full of great tech hampered by high cost.
Foldable phones, however, have been around since 2019 and Apple is entering the market when foldables have matured to a point where they're just as thin and lighter than an iPhone Pro Max. Many folks buy the iPhone Pro Max because it's the biggest display Apple offers, and now there'll be an option for an even bigger screen that still fits in your pocket. That offers a lot of appeal.
Other thoughts:
1. Foldables actually utilize Ultra-Thin Glass with a plastic film screen protector. The inner screen on my current and past foldables feels great and I use it to write work meeting notes.
2. The hinge on modern foldables is remarkably solid. All of my foldables since the Fold4 have felt very solid and well-made. I don't baby my foldables and use them just like past slab phones and in all types of inclement weather and the beach.
3. Performance has been a non-issue because they come with flagship SoC with ample memory, and the software is optimized to make the foldable experience shine.
4. My past Fold 4, 5, and 6 were easy to use one-handed, even at Costco while pushing a cart. Heck, I've even unfolded my 10-inch TriFold while pushing the cart. The balanced weight helps a lot. My iPhone 12 Pro Max was actually harder to one-hand and always wanted to tip out of my hand because it was so top-heavy.
I expect the iPhone Ultra, or whatever it will be called, to really drive the foldable market and increase mainstream adoption. It is more expensive but the cost is easily managed through trade-ins, people selling their iPad mini, and installment plans. I just hope Apple updates iOS to take full advantage of the foldable features and experience.