IMO the default should stay lua5.1 for one release to allow packagers to go through the process of changing dependencies. For example I think Debian requires an approval process to change them. Also "somebody" needs to integrate it into the Windows bundle.
We can document that it will change in the release after and as soon as the next release happens we can push the change of default. Of course distros that want to change immediately can just set the CL option to luajit.