After several versions of Rhino, I wanted Rhino 6 to work in this kind of situation.
Extrudes Tapered generate triangular surfaces that do not allow the shell command to succeed.
It would be a surprise that these kinds of examples would be solved, because rebuilding all the surfaces is a very expensive job for the user: a waste of time!
Shelling not work.3dm (340.6 KB)
(Rhino is a powerful tool, but often, it drags from version to version known issues not resolved, or partially solved.
In my opinion, it is more important that Rhino succeeds in performing such (banal) modeling operations, which will surprise you with crazy renderings or views!).