I suspect that Cessna has run the numbers and realizes that the cost of
a
"clean sheet" light airplane would simply never be recovered in today's
market.

Agreed, but Diamond nonetheless managed to come up with 4 "clean sheet"
designs in less than 10 years - including a light jet. Why can't Cessna
and
Piper do it? Is the FAA THAT much worse than the JAA (or whatever the
european equivalent is)? You also have to wonder how much longer Cessna
and
Piper can afford to NOT bring out a new design.

Yes, the FAA is that much worse. Look at the trouble Diamond is having
getting the DA-42 certified in the US.

A lot of Cessna dealers are wondering when Cessna will do a new design.
There are rumors, but the fact is Cessna does not really seem to be all that
enthusiastic about manufacturing the airplanes it has.