I think this is where the tech going into the iPad Pro and iPhone Air is destined to end up, if they can get the thickness of each side into the low 5mm ballpark, then a 10mm folding phone vs. a 8-9mm pro model and its not much thicker.

I’d trade 1-2mm thickness for a 5.5” form factor (please submit your jokes below)

Well the foldables (like Galaxy Fold 7) are already under 9mm. So being the size of two phones days are over.