Hi,
The assumption was that (at least one of) the pingers worked, so if sat on the abyssal plain, they would have been heard.
It's not that assumption you wait from professionals.
They know that pingers can fail ... or be destructed for any reasons
In a investigation you must be open at all options and exploit them
If you base an investigation on dogmas and faith .. it's the best way to fail.
So .. this assumption is even more frustrating for me.