Deanm - good observation.
Building on that, why not incorporate GPS data as a fall back, in the event of a pitot head icing up, the FMS advises that there is a mismatch between GPS/Pitot speeds and does nothing pending pilot acknowledgement.
Descent to a lower level at this stage gives more speed margin whilst reducing a stall scenario, the secondary effect of pitot head icing is loss of awareness and subsequent loss of control (in some cases).
Just thinking about being in heavy turbulence/wind shear at night with no speed awareness and close to speed margins gives me the sweats.
Prevention is better than cure.