>end up with UEFI > NT6 bootloader > Windows or Linux

I don't see any advantage to involving Microsoft in the boot process.

>paying attention to the built-in NT6 bootmenu

UEFI firmware vendors provide a boot manager (the UI) for choosing what installed OS to boot. Apple has offered a graphical boot manager (option key at the boot chime) since forever.

NT6 isn't free or open source software so I don't see the various Linux projects wanting NT6 involved at all, in either the single or dual boot use case.

But in effect you're proposing different boot paths, and different UI/UX depending on whether the system is Linux-only or dual boot Windows Linux.

>This deficiency was not a factor before UEFI struck

I can't tell what deficiency you think UEFI has that BIOS didn't have. But maybe you don't like specs? Or are you referring to the much latter added UEFI Secure Boot?

>No matter what, there's not supposed to be any need for a shim.

You seem to think Microsoft wants to be in the business of executing arbitrary and unsigned code, despite all efforts proving the opposite.