There has to be a trade off between safety features and system complexity and with this a certain level of pilot competence is expected. Probabilities feature largely in how systems are designed and what protections are required
Having said that, if the type of deep stall that has occurred here can render the normal stall warning useless due to airspeed < 60kts, would it not be possible to derive a warning from a combination of high rate of descent / Vertical speed together with a high nose up attitude above a pre-determined or abnormal value?
Its possible that incidences of deep stall do not justify this change to the normal warning / protections.