"Upgradeable" is too big of a word here, specially considering that they're using different form-factors even between the models released on the same year (e.g. pro vs non-pro) ; and also different from models released on the previous year (e.g. studio). This almost certainly means that next year's model will also use a different interface, so you won't be able to upgrade your storage at all.
You might be able to. You just need to make sure you get a compatible module somehow.
I wonder if 3rd parties will start selling them. If the memory controller is in the cpu, there’s no reason for the little board housing the ssd to have any proprietary chips…
Depending on your level of price sensitivity, you can always use a Thunderbolt SSD, an external RAID array of SSDs, or just get the 10 Gigabit Ethernet upgrade and hook into local NAS.
The M4 Pro Minis support higher capacity modules, so it's not too shocking that they are not identical.
We've already seen videos from the usual suspects showing that people who are sufficiently skilled with a soldering iron can replace the flash chips in the modules with higher capacity chips, in addition to replacing the whole module.
As noted above, you can simply replace that module with a higher capacity module with just a screwdriver, as iFixIt did.
However, there is a real opportunity for those who do have soldering skills to make a quick buck here.
You could pretty easily buy the cheap base model M4 and resell it as a custom upgrade build, as long as you were clear that the SSD was no longer stock.
It's not an easy solder job and they are picky about what NANDs they work with and how they're configured. It's better than soldered to the board but not by much.
Sorry, but again this is an abuse of the word upgradeable.
You could do the same with many other laptops and even some phones out there - buy from a 3rd party who has resoldered the corresponding parts. The replaceable modules brought you nothing.
Even the economic motive you mention is actually just because Apple overcharges for storage, and has nothing to do with the replaceable modules. As long as there is no cheap 3rd party source of these, a replacement module ecosystem makes no economic sense (someone will always have to bring in his device for resoldering, or you lose the price of one good working base model).
