save a single float from 1.6 which is not without bugs to have a 1.5 load it.

This is forward compatibility. Not backward compatibility !

It make no sense if an error can be fixed by having user edit one single character in weight file and absolutely everything works .

This happens for most library that use version for their serialization format. The library can only load the a given set of formats. It won’t be able to load the future ones.
Is there a specific reason why you want to be able to do this?