Building X is probably going to be trivial going into the future, yeah. Building and running future apps written for the Wayland will be a different beast though.
Just like there is XWayland today to make it possible to use a Wayland system with X apps, there are already similar solutions to run Wayland apps on an X system.
If you run `cage`, it will run a Wayland application in its own sandboxed window while the rest of the system runs X.
Similarly; if you run `sway` under X for example, it will start the window manager in its own window, and you can run Wayland applications inside it.