I’ve had some experience in underwater searches, albeit more body searches in rivers & lakes. The problem here is that underwater searches have to be thorough. The goal is to be able to say, “we searched this area and it (whatever ‘it’ is) isn’t there.” So you push you equipment to it’s design limits if necessary. In the case of a robotic sub you can go beyond the operational limits without risking human life, so why not?
You do understand that the sub is designed to ‘fly’ a pre-determined distance above the ocean floor? This means if the ocean floor falls away abruptly that the sub ‘dives’. If the sub goes beyond a pre-programmed depth where seals can fail, it’s programmed to drop ballast and return to the surface.
It’s very possible that this aircraft will never be found. The ocean floor is over 3 miles down. And if that aircraft hit the water hard, as I think it did, it’s nothing more than a couple of engines and some razor blade size pieces of aluminum spread out like confetti over miles of the deep-sea floor. Not even a decent sonar target without the Black Box pingers.