The PC was only a success for the market as a whole. The setters of the standards did not profit from it at all and even went (close to) bankruptcy because of it.

Contrast this closed hw architectures like Nintendo and Apple produce. No consumer freedom but incredible profit margins.

Note that modern macs are in fact PC's with just minor modifications so they profit from the PC economies of scale while still locking their customers into their hardware platform.

What if lockdown worked in such a way that you could lock it down to only run your Linux kernels? E.g. load up your own certificate in the TPM and use that for signing when you build.

In that case it would be a security feature -- another line of defense against bootloader malware and/or adversaries in physical possession of your machine.

(I don't know how technically feasible that is; I know Canonical and others are looking at having their own cert so at least their unmodified kernels can run, but I don't know the mechanism for how that interacts with already-released UEFI machines.)

The point is that technologies like this are a double-edged sword, not evil in themselves. A similar argument is made by Linus himself for sticking with GPL v2 instead of moving to GPL v3, which outlaws certain DRM-related uses; he's more interested in providing a functioning mechanism, and leaving the policy-setting to others.

sadly the success of locked down.apple devices stands orthogonal to it.

The PC was a success mostly for Microsoft. With the commoditization of hardware around a single software option, PC makers were squeezed and their margins are exceedingly thin. It's the "thin slice of a larger pie" metaphor. It's just that Microsoft has the whole OS pie, while every hardware maker has a vanishingly thin slice of it.