DAPT;
Mayday calls have saved many lives. If you can't lower the collective while making a radio call when you are already on frequency you need to be retrained or find another line of work.
How would a Mayday call have saved any lives in this incident?
I think the scenario presented isn't one as simple as lowering the collective and getting a call out. All well and good at 1500ft above an airfield, but over a city, both engines unexpectedly stop, at night, in forward flight, picking a landing point, chasing the rpm etc.
If you're so hung up about the distress call not being made in this incident, remember, neither did the Malaysian 777, despite apparently being able to change heading!