Retrieving FDR/CVRs in deep waters has always been difficult. If the orography of the sea bed is what has been reported ( the " Alps" with 4000m deep crevasses) , then the task will be even more difficult. Pingers or no pinger.
Incidentally I was reading yesterday the (excellent) interviews of both the pilot of the Cessna 210 that ditched with 6 POB off the coast of Corsica on 12 October last , and that of the Puma helicopter Pilot who finally found them .
(the C210 pilot was an AF 777 Co and his partner a Flight attendant with some experience ) The C210 was on radio contact until 1min prior ditching, , called Mayday, was under radar contact on SSR transponder, and had a new 406 Mhz ELT coupled with GPS. Rescue SAR knew exactly where the a/c went down. A first helicopter went above that point only 15 min later but failed to spot the survivors.,despite all on orange life vests , but drifting. This was 1400 Loc , full daylight. 3 rescue helicopters and a Breguet Atlantic joined the rescue. They never spotted them , finally when night fell 5 hours later, the C210 pilot activated the small light on his life jacket and that was picked up by an infra red camera of the military helicopter , who got them all 6 out.
Hypothermia, small bruises but all OK.
So even with all the new gear , ELT and all, at sea, it seems extremely difficult to retrieve something floating ,even knowing the exact GPS position 15 min before. let alone FDRs/CVRS at 4000m deep in an area 30 x30 NM months afterward...