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?"

Exactly my point. Let's say it drives by telepathy. The car wouldn't change much: it would still have a driver and be driven on the road. It would still have an engine, passenger seating, storage space and cupholders. It would still at a glance look like any other car. But that apparently small change would result in a profound change in the user experience (and yes, profound engineering challenges for the maker). It's a better car. But it's not an entirely new class of transportation, just a tweak to an existing one, like the automatic transmission and hybrid engines.