Are there any Arm cpus from different makers where the same packaging is used and enough of the pins are the same that you could use a common subset of the IO facilities offered by each chip?
I recall code to run on both Intel x86 and NEC V30 clones had to avoid using instructions that were specific to just the NEC clone.
Second sourcing seems to have almost disappeared now (as have many of the former smaller manufacturers either defunct or merged with others).
Here is a blast from the past for those who haven't seen it before -
Captain Zilog Z8000 takes on Dr Diabololicus (aka Motorola & Intel)
They didn't win and ISTR only Ollivetti with its M20 machine ever made a PC based on that chip. M24 was Intel based since by then they had won!
Sinclair's QL did nothing at all for Motorola chip sales even though the machine itself was quite nice the microdrives were truly horrific. Who wants to own a computer with write only storage media?
It was vastly out sold by Amstrad's PC512 8086 box which came out at a couple of years later with a real floppy disk drive.
Was there ever a successful mass market machine 68k based?