I think the reason why GNOME did not make the GNOME Classic a fallback is
they did not want to compete with GNOME Shell which they hope people adopt.
The what-if-you -had-a-party-and-nobody-came syndrome. :) Also they wanted
throw all of their resources behind the Shell. Apparently there was some
push by developers chiefly from Redhat to keep GNOME classic, but they
worked on GNOME Shell against their better instinct because that is the way
RH operates. They respect downstream and go with the flow.