But it's important to note that these days, there will always be somebody who wants to bring their code in at a lower level. And the immediate reaction of all vendors is "But why would you want to do that?". Why would you want to bring your own Unix flavor to our hardware? Why would you want to run Linux executables on Chrome OS? Why would you want to run a different hypervisor on Oxide racks?
Those people will exist. And layers like BIOS and UEFI allow system vendors to say "Eh, sure, we don't think that what you're doing is wise, but whatever, here's a standard way we'll allow you to use". Holistic systems don't have these standards. They are only usable as their creators intended. And not everyone will agree with their creators intentions. But the work required to refit them to fit another purpose, which the hardware is very fit to do, but the software fights against, is usually too high, and you'll be better off using something that is worse, but left you an avenue to customize it for your purpose easily.
My main takeaway from this talk, is that we need some standards for phase-based booting, as there is some very cool improvements that could be done with it. But I see no need for holistic systems as described in that talk, I just see a need for better, more transparent systems. And a system doesn't have to be holistic to be transparent, or good.