I made this proposal 6 years ago and in the meanwhile I'd simply close it as "won't fix".
With a little effort everyone can run this "runtime" model on his own. In practice we use a common parent anyway where all dependencies are defined with their versions (and sometimes also scope) in a depMgmt section. That will effectively overwrite also everywhere the transitive deps. By adding runtime scope for all those dependencies you will get the requested behavior and require every project child to overwrite the scope again if necessary.
In practice we did not do it in the end, because Java sometimes requires compile time if a component exposes types of a transitive dependency in its own API. Our prominent example was Apache commons-lang. We ported our code everywhere to commons-lang3 and wanted to set runtime scope for the old commons-lang to avoid accidental usage.
However, as it turned out, it was not possible, because we used also commons-configuration that throws an own ConversionException derived from an Exception in old commons-lang. In consequence the old commons-lang had to be visible for the compiler also.