God forbid OpenWrt ever receives an update or something.

You know, just because it's in an RFC doesn't mean it's actually practical. 'home.arpa' is significantly worse than every other option.

An update wouldn't change all the existing configurations.

If you actually think it's a trivial affair to change a well-established default with more than 20 years of history that is, on top of all other difficulties that such a change typically encompasses, used to identify and name things, I hereby beg you to never design or provide any kind of infrastructure.