"why not? legacy mtl is not going anywhere. another option I see is asking devs for additive blend mtl.<"
how you know that? are you official from the corona team or have official infos?
will it work on the chaos cloud, or in vantage/enscape connections etc in future?
how does it go with vray 5 materials (and possible later exchange back and forth to vray/or overall in CHAOS eco system (vray 5 has kept the full reflection controls)?
i really think it is not correct to take away this controls of a modern material. it should be a tool for artists.
the new material isnt fully physical anyway, as colors you can use, and other % controls etc are there, which arent physical at all (the whole "PBR" thing isnt fully physical at all, which is ok).
overall "jojorender" brought it good to the point