You are of course correct for the initial search, but at some point you hit diminishing returns. Even if the failure were a technical one, the value of locating the wreck and determining the cause is likely of limited value. There are only so many systems that can fail, and we already do thorough failure modes analyses when designing aircraft. That's why flying is so safe these days.
The 777 has a pretty good track record with 1,212 units built and five hull losses, only two of which were due to failure of flight systems. If the hull cost $261.5M and you estimate the value of a human life $10M, then the MH370 incident had a base cost of around $2.6B. If you only had one failure out of 1,212 hulls, that suggests you'd be willing to spend 0.08% or ~$2.1M to make sure it doesn't happen again.
This is just one formulation of the cost/benefit and of course excludes some important factors like the public relations cost to MA and airlines in general, but hopefully it illustrates that there's a bound on how much we should reasonably be expected to invest in understanding the events of the incident, and that it is not an absurdly high value.