Its back to the old problem of the false warning rate. If an engine shows a dodgy HUMS, you just shut it down no big deal. But if a main transmission shows a dodgy HUMS you have to ditch, and would look a bit stupid if it was a false alert. You only have to look at the maintenance actions reported in the May ditching to see that even the manufacturer suspects a false warning as the most likely cause - viz the "replace the sensor mounting washer" action. With current technology having cockpit warnings of main transmission problems would dramtically increase the ditching rate.
I am not saying that improvement in self-monitoring of the HUMS system itself and therefore increase in the system integrity is not possible, but it is an area that has received no investment and so no progress has been made since the early 90s when HUMS was invented.