OP wrote:
"I hope the Right To Repair movement goes nationwide and things like this can't be done by companies."
"Right to Repair" isn't going to change anything here.
You can still "repair" the m4pro Mini. I wouldn't be surprised if Apple will provide the parts and instructions.
What you CAN'T do is "swap" the drive from one Mini to the other. That's not really a "repair", because nothing was "broken".
I believe it has more to do with the way the m-series hardware and software work, than the physical drives themselves (although I believe there are differences between the SSD modules used in m4's and m4pro's)
You could still buy a 3rd-party SSD upgrade for the m4 Mini and install it, if that's what you want...