Wayland has been around, I think, at least 3 years before Mir's announcement, though. It's had plenty of time to evolve naturally without any fixed corporate goals necessitating them to act fast.
It's also worth noting that many of the changes required when switching out the display server (moving stuff out of X and into the kernel, adding features to drivers) was done while developing Wayland. Mir doesn't have to worry about this stuff, so they should be able to do it much quicker than the time it has taken Wayland to mature.