I hope they convince nvidia, but on the flipside, hope canonical has to maintain all of their Mir support out of tree for the various toolkits. (no one even commented on the MESA mailing list about their Mir patchset, hopefully this is a sign that upstream doesn't appreciate what Canonical is doing with Mir). If that is the case, i hope GTK+ and Qt do the same.
I can not see way qt should have anything against uppstreaming of the mir support. Multiplatform is a core idea for qt and as I understand it you can usually do it in a very clean way.