That said, I have bought many cheap Windows PCs/Laptop in my lifetime and I have only ever upgraded them once and they also don't last as long. Somehow... I don't feel shouting at Apple. These things do last a bit longer...
Soldering them to the board is just an asshole thing to do though, especially since these machines can't boot off of USB if the NAND dies. Surely some elastomer BGA sockets wouldn't cost that much. There's no sane explanation other than they're doing it so you have to buy a new Mac to get more storage.
The increase in per unit cost probably would be entirely insignificant and minuscule compared to the revenue they'd lose by not being able to charge predatory prices for storage upgrades. So it would be a secondary or a tertiary concern at best..
What they/everyone really ought to do is to standardize that, with the flash chips themselves still connected via a modular connector and the "NVMe controller" as open source.
Imagine integrating the flash ECC/RAIN with ZFS et al. Or the ability to decide for yourself if you want a lot of QLC or a bit of SLC or a mix of both, in software at runtime.
At least some portion of an operating system's bootloader chain must be installed to the internal storage, because that's all the firmware knows how to read (unlike Wintel PCs where there's a UEFI driver providing USB storage stack support). That bootloader running from internal storage is then free to enumerate external storage devices to locate the rest of the OS it is trying to load.
Irony, of all laptop I have owned, the most problematic was the Apple PowerBook. It's screen became defected a month or two after the warranty ended. The external VGA connection had issues and might require a couple restarts to get a signal. It could barely be used as a desktop computer. Even though I used it to write my first production software solution. It was ditched as soon as financially possible.
Apple, the richest company in the world, who spends millions in money and engineering hours on stuff like making sure the packaging having the right neutral smell, and the box sliding out with the right amount of friction when you open it, and on security teams/mercenaries able to pull family members of workers from warzones, and you're telling me they have to nickel and dime their HW team for routing an NVME slot on the board instead of soldering the NAND chips because that would cost some more engineering time?
Thanks for the chuckle, I loved it. I think Apple spends more on toilet paper or hand soap in a month than the effort would cost their HW engineers to do that.