I believe the closest we've ever come to seriously proposing a feature closer to traditional inheritance is in the "delegation" discussions, but I'm not aware of any delegation proposal that ever proposed using one of these four keywords, and the current RFC proposes only adding new delegate and to keywords. So I have to agree the odds of us ever using these four are pretty much nil.
Plus, now that editions are a thing, we could re-add these later in the one in a million chance we'd ever want to.