All versions of the MacBook Air appear to have upgradable storage, even though none of them have upgradable RAM.

I base that on MacSales.com offering storage upgrades for all 5 generations of MBA.

That's actually okay for me, because that's something the customer benefits from, since it uses less space which is crucial for mobile hardware. Soldered RAM and possibly HDDs don't serve an obvious customer benefit and if they do, Apple should communicate them.