Sure, as long as there’s a chance of finding that airliner in time, we owe it to any possible survivors, the families, and airline safety in general to do our best to try to locate it.
But when you haven’t a clue as to where to even start looking, when they’re going to be all dead anyway, what’s the point? If it fell at sea, without electronic aid (which appears to have been disabled) there is no way to locate it. Even if that debris spotted by satellites off Australia turns out to be part of this aircraft (highly unlikely) that still tells us nothing of where on the sea floor the black boxes are, or whether they can be recovered in time. Assuming they do haul aboard a recognizable fragment of wreckage today, the rest might be a hundred miles away in two miles of water. It might be years, if ever, before it was even found, much less brought to the surface.
There’s also the matter of cost. Ships and aircraft devoted to this search are extremely expensive to operate, and their crews are in danger. Sure, some of it could be written off as training, but is it worth losing a P-3 in the roaring forties for such a slight chance of success? That merchantman that was diverted from its delivery is burning a lot of fuel and paying its crew a lot of overtime for no good reason. I’m not being facetious, mariners will gladly take a severe financial loss to carry out a rescue, but if there is no rescue, the loss will still be there, and someone will take a bath for it.
And think of it another way, how many aircraft could be equipped with continuously reporting EPIRBS (if yachtsmen can afford them then certainly airlines can) for what this rescue effort spends every single day?