"System builders who'd like to play with an M1" doesn't seem like a relevant or sufficiently well-defined market segment to justify such a huge trade-off.
Pretty much
No one in their right mind at Apple would ever open up their tightly-integrated (ie always "Just Works™") hardware stack for outsiders to "play with"
Want to "play"? Buy something else
Apple doesn't care what else you buy - they do care what they sell (and support)
If they opened-up the M1 (or any of their other proprietary products) to outsiders, they'd be on the hook for supporting whatever insanity other manufacturers did to/with it
I remember that they caused hardware sales to drop due to the clones being cheaper and the money made from the OS license did not make up for the shortfall.
Anyway I have Zero data to back this up. But I remember reading about this years ago.
You're right - it only makes sense to you, though I'm open to being convinced otherwise.
Nothing about port selection has anything to do with the M1 chip.
Apple is clearly not interested in the builder market at all.