Even if we ignore the fact that Windows RT is long cancelled, has anyone ever seen either in the wild on any machine?
Ultimately my point is I don't think the following is true. What you describe basically exists, and no one wanted it. For Windows RT I remember it basically being, "wait a minute, this thing can't run my normal apps".
> but that might upset legacy corporate customers so instead they let ordinary people get hacked or lose their family photos and spend money on repairs
I doubt Microsoft could manage to market an umbrella in a rain storm, so I imagine that's why S mode is basically stillborn. These days people use a lot fewer native apps (and now it is possible to package many of them into the Store), and S mode can be converted to full. I imagine if it started in S mode by default most people would never notice