I agree that's a proper description of why Microsoft, etc, didn't do touch first, but what exactly is that relevant to?

Your contention earlier was that stylus -> touch was a "tweak." My point is that it's not just a tweak, it's throwing out the existing model and starting from scratch. You implicitly acknowledge that by referencing the investment to make the change, an investment that is incurred by having to throw out the existing design and start over.

Touch was not a tweak. It was throwing out the steering wheel and asking "so now how do you drive the car?"