The sunk cost here I think refers to the existing CPU designs of the respective camps. Qualcomm's ARM-based cores don't support an equivalent of the C extension and adding it would presumably require major and expensive rework.
>> This is basically what qualcomm proposes, 32 bit instructions and 64 bit aligned 64 bit instructions.
Well that's only sunk cost if they assumed from the start that they were going to change the design to RISC-V AND drop the C extension. In that case, it was a rather risky plan from the start - assuming they can shift the industry like that. I'm guessing RISC-V was a change of direction for them and this would make things easier short term.
