> will imply changing every single one X app out there.
Maybe it depends on what you mean by an "X app".
You shouldn't need to change most Gnome and other GTK apps. GTK already has basic Wayland support.
You shouldn't need to change most KDE and other Qt apps. Qt already has basic Wayland support.
So all these should run natively on wayland.
I suspect TK and wxWidgets will also provide native support if Wayland shows any sign of success. Maybe they already do.
If those are "X apps", then you don't need to change every "X app".
If they are not "X apps", then ... how many "X apps" do you use? xclock? xload? Certainly there are some, but they shouldn't be a big barrier to change, assuming the change brings any value to makes the cost worth it (which I think it will).