They also have a new T2 chip, just like the new Air, and every incarnation of the T2 seems to include the SSD controller for the disk that provides inline encryption, tying the chipset/storage more deeply than before. I'd say it's the same here. (This maybe isn't a strict technical limitation for not making the disk replaceable, of course, but I'm just speculating it's part of the reasoning.)
I'd guess that, unlike the SO-DIMM change here, Apple will continue to vertically integrate their storage solutions across all products as they roll out things like this. So soldered NVMe is all you're going to see from this point on, I'd imagine.
Of course it's not user expandable, how they will make money. Very few people will buy 128GB version and SSD increase will have very premium cost, like usual. But you should be able to use external disks via thunderbolt if you want to.
There's really no excuse for that these days. So disappointing that Apple must bake in this kind of greed into the product. Overall I think it's shortsighted because it will drive away many budget conscious customers.
