KTM 11, there is only one legal requirement for a functional WX radar on aircraft and this is tailored by every manufacturer for the specific type. Companies publish this requirement in the MEL or DDM (as applicable).
This "law" is only valid before commencing any flight.
When airborne it is the crew which has to decide the best course of action, and yes, there is the possibility that a return may be the best action.
I doubt that the AF crew had a chance to consider this option at all. With all details available here it appears that the wx radar was a "minor" problem they were dealing with.
Being sure that the crew followed law no. 1 precisely, "Fly The Aircraft", they faced a situation not covered in any QRH or manual.
It does not appear that any action they went for helped at all. Even well trained crew, unfortunately, can and will face unrecoverable situations.