Search to take decades?
On whose dime?
This question is based on the practical matter of effort versus benefit. If the estimation is that the aircraft was in control and that a human agent was involved, is it really necessary to find the aircraft to arrive at some means of preventing such incidents in the future?
At some point, when dealing with the human mind, you can't engineer a "solution" to how people think and behave.
Looking for various loopholes that may have been exploited does not IMO require finding the aircraft on the ocean floor.