Any desktop computer would have to be redesigned to add a "allow new device" button since they have no other input.
Even on many laptops, the internal keyboard and mouse are USB devices, when you install a new OS, do you have to accept trust to those as well? Or how will you stop an external device from spoofing them with the same vendor/device ID?
This sounds like something that creates a chicken-and-egg problem of there not already being any such DTLS-speaking USB devices... but how about if vendors just create a little USB dongle that wraps whatever's plugged into it in "authentication" using DTLS? Ship the dongle with the laptop; tell people that if they want to install a new OS, they have to plug a USB keyboard in through the dongle.
hell, simply through acquisition and acquiescence, the market already accepted locked-down platforms. at this point, we ought to have more benefits from this instead of just making these platforms hard to install Linux on.
Or only allow completely unauthenticated devices as a fallback when there is no other available authenticated device.
A computer not having any keyboard is a rare case. Most of the time you have what is built-in (and should be authenticated) or what came with the computer (and should be authenticated).
Allowing unauthenticated keyboards only on detection of no authenticated ones probably covers 99.9% of all use cases and increases security dramatically.