I boot UEFI, but have SB disabled, so this will have zero impact on my systems.
I have a Mac mini 2006 (upgraded to 2007). I compiled GRUB 2 for 32-bit EFI and it boots 64-bit FreeBSD from ZFS just fine :)
So I guess it boils down to "it's not broken for me, why change it?"
As for motherboards, I doubt they'd get rid of it that soon, either. If only because having it there means that OEMs can ship devices with FreeDOS preinstalled to dodge the licensing fees. As I understand, legacy mode is basically just an emulation layer on top of UEFI, and it's fairly cheap (and already there).
This is different for mobile and tablet devices like Surface, which aren't really advertised as general-purpose PCs in the first place. Since they're only expected to run whatever's preinstalled on them, so long as it only needs UEFI, they will likely ship with UEFI only. But I'm not running Linux on such things.
And I don't consider a few seconds to be "painfully slow" for something that is done maybe once a week, if that (suspend is there for a reason...).
As far as "messing with MBR", why is that somehow problematic? MBR was specifically designed for this exact purpose - hosting a partition table and a boot loader. And there are tools to do precisely what needs to be done to it, and these tools have been there for a very long time - so I'm confident that they work right. On top of that, MBR structure is simple enough that I can manually inspect or edit it with dd directly on the block device in question.
* http://jdebp.eu./FGA/efi-boot-process.html
What they've done with the "disable" option is enable a compatibility support module for UEFI to present a faux-BIOS to the operating system. The point of a CSM was to provide backward compatibility to legacy operating system that have no idea how to talk to UEFI, but can interface with BIOS.
So all you're doing is using UEFI and also a fake BIOS layer. But they call this "disabling" UEFI. Hence it's a big fat fucking lie. The things liars tell people who they think are stupid. So, you know, you should be a little pissed at people who think you're a moron that you need to be lied to.
Ohhhhh but we were just trying to make things easier to understand, we were just trying to be nice!
Well, guess what? Go fuck yourself. Stop being nice. Try being honest.