There's so much more to a chip than its ISA. I'm not sure how the ISA changes anything. If I design a board around an STM32, for instance, even within that family I'd have to find a chip that has the same exact footprint to be able to replace one that I couldn't find. And even then, I'd like have to reconfigure the software ...
So unless there was a pin-for-pin/electrical standard for chips, the ISA is of no effect.
So unless there was a pin-for-pin/electrical standard for chips, the ISA is of no effect.