I raised this originally on the WHATWG mailing list and there was not much discussion there or different opinions. I think the change makes sense if we are to keep the online whitelist which I believe we do want to keep.

The suggestion is to give online whitelist namespaces that are not a single * character priority over fallback namespaces in the "Changes to the networking model" section. This allows you to give everything a fallback using the / fallback namespace except for a few online whitelist namespaces listed in the online whitelist namespace section without requiring fallback entries for them.

I have not heard a drawback to this proposal so far.