Firstly, Ubuntu covers about a third of desktop Linux installations. That means that majority of desktop Linux users use something other than Ubuntu. Even on Steam, Ubuntu doesn't have the majority of users. Ubuntu may have the biggest single market share of any distro, when talking about home users desktop installations, but that means nothing.
Meanwhile, Nvidia/AMD don't really care about Steam or any of that, not at this point anyway. Maybe in the future when the market grows, with steamboxes and other Linux consoles coming to market, but right now? The real Linux money for GPU makers is in render farms and workstations. That means mostly RHEL, Suse, and the like. Not Ubuntu. RHEL will use Wayland, so when the render farms move over to Wayland, at that point, Nvidia & AMD at the very least are going to offer drivers for them. They'll have to, there's just no question about it.
Secondly, there's not going to be any "Mir drivers". There will only be EGL drivers, which can be used by both Mir and Wayland. Mir is not going to ever be used by anyone other than Canonical, because no other desktop environment is willing to adapt it, they're all going to Wayland instead. There are very good reasons for this, you could ask the KDE devs, such as Martin Gr??lin about them, or you could ask the GNOME devs, read their blogs or whatever. But it boils down to the fact that Canonical is developing Mir for the needs of Unity only. They are not thinking about other distros or desktops. They're not creating a stable protocol, they're not enabling others to reimplement Mir for themselves, they control the development and do it all in-house, without collaborating with others. This all is contrary to what Wayland does: Wayland provides a way for everyone to implement their own compositors, while providing a stable protocol, compatibility, and a development model that caters to the needs of every distro and desktop environment.
Thirdly, even in the extremely unlikely situation that the GPU makers would only offer "Mir drivers" (which, again, will not happen), but even if it did... it wouldn't matter, because Wayland supports multiple backends and runs on pretty much anything, all you have to do is implement the backend. It can already run on EGL, Android drivers, or plain CPU rendering (Pixman). It would be pretty much trivial to write a Wayland backend to allow it to run on those "Mir drivers" (which, again, will not happen).
Fourthly, Mir is a totally useless waste of resources. There's nothing it offers that Wayland doesn't. There are no technical reasons for its existence whatsoever. The only reason it exists is because Canonical wants to separate Ubuntu from other Linux systems. That's pretty much shooting themselves in the foot, as an OS that is 99% dependent on community-written code, meaning that if the community suffers, Ubuntu suffers also.