What stops working
When the prototype is the product and producing a screen costs nothing, design-as-production — the hand-assembly of artifacts as a step upstream of engineering — loses its scarcity. What does not lose value is design-as-behaviour: interface quality, accessibility, information architecture, and interaction detail are not decoration on the product, they are the product the user actually meets.
What it becomes
Design Thinking's discover-and-define front half is where the outer loop earns its leverage, and it is precisely the part the machine shaft cannot supply for itself. The work is framing the problem, choosing which user to believe, and arbitrating the trade-off — judgment as torque, not speed. Software development's best-kept secret stops being deniable here: it was always a human-behaviour problem wearing a technology costume, and design is where the costume comes off.
The strongest objection
That craft and taste still live in the execution. Conceded — and only half-answered by moving upstream. Taste now expresses itself in what is specified and what is let through, and still in the lived quality of what ships. It relocates toward framing; it does not vacate the execution.