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We can't just have planes inexplicably disappear somewhere never to be heard of again.

How much do you think is an acceptable cost for the search? The case is very unusual, but I suspect that the authorities have a VERY good idea of what happened, even on the basis of the detail that has been publicly released (they will of course have more than that). However, without the flight recorder and other aircraft evidence they can't prove their case against the extreme outlier possibilities, so they have to keep quiet.

So what we're looking at is taking the $150m spent so far, spending more money still on a further search, in which there's no guarantee the wreckage will be found, no guarantee that they'll find the flight recorder, they certainly won't get much biological evidence from the bodies, there's no guarantee they'll recover the flight recorders, no certainty that they will contain useful and recoverable data. And from the wreckage so far recovered we know there's little chance of recovering many bodies. Note as well that Ocean Infinity have offered to try and find the aircraft, not apparently to do any recovery work.

Is (say) another $100m a worthwhile investment, when there's no reason to expect that this will offer any closure in terms of finding the cause?