The SecureROM boots iBoot1 from NOR flash, then that has the SSD driver code. It would certainly be possible to add support for external storage, as long as it still fits in NOR. But I doubt they will.
My take is the company deliberated about this trade-off quite explicitly at some length and decided the Mac serves the world in its current capacity as a "computer" (i.e. the truck in the truck vs car analogy) and that they do not wish to limit the capabilities of the existing Mac that people love in any shape or form by moving to ARM, which was highly speculated and ripe for potential backlash. They probably decided the Mac would be an "open" system to some degree (at least as open as it already was) and iOS would be the closed mass market computing device optimizing for security and dependable end-to-end experience.
Likely there is a very small bit of bootstrap code stuffed into a ROM somewhere, and the only thing that bootstrap code enables it to read from some protected part of the onboard SSD, which then gives you the next round of bootstrap enabling you to read from other devices (e.g. all the code needed to power up and use the hardware needed to get to an external drive, and the code to read the partitions on said drive).
Someone made the decision that it would be better to use the bit of internal SSD (since it would "always" be there), that could be changed later, rather than hard-code this into comparatively expensive silicon. Unless your internal drive goes bad, it is a pretty good compromise. I seriously doubt that anyone in marketing cared about this.