I would be surprised if distros got rid of it anytime soon. There's still a vast amount of relatively old non-UEFI hardware that people still use, and it's not going to suddenly change, because that hardware is plainly "good enough" for all practical purposes - so it's only going to be replaced if and when it fails. Even Windows hasn't so much as announced a switch to UEFI boot only, and Linux generally supports old hardware for much longer. So I wouldn't be surprised if it's still there in a decade - maybe not in all distros, but at least in some mainstream ones, especially more "hackerish" like Arch and Gentoo.

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.