Unless I’ve misunderstood things in a number of instances, package object inheritance is currently the only way to get import-free implicit prioritisation. So assuming that is the case, I hope package object inheritance will not be removed or deprecated before an alternative method for import-free implicit prioritisation is put in place.
I also feel it might be good to agree on terminology. I’ve used the term type-classer. Some type-classes have an associated type-classer extension class, Eg there is a Shower implicit to give Show extension methods for Shower objects. In the case of Show just about Any object can be a Shower but that is not the case for all type-classes. But there is no equivalent Unshower extension methods for the Unshow type-class.
Shower itself is straightforward and doesn’t need implicit prioritisation, but other type-classers do. It is these implicit extensions for the type-classers that seem to require package object inheritance to be import free.