Re: It is a salvage mission in international waters, China should take over the search
No problem is suspected with the engines or airframe - given how long the plane flew. But although one theory is pilot action, electrical problems are also very likely. Fire or decompression, leading to oxygen-starved pilots not acting rationally has happened before.
But even if it is a pilot, there may be things to be learned abou that which are useful.
The point is, this isn't wasted money. The reason that air travel is safer than almost any other form of transport is that we put serious money into finding out what went wrong in the past, so we can solve it in future. Even some suicidal/homicidal pilot problems may have useful solutions/mitigations, if we know enough.
It took 5 years, and serious effort to find the Air France crash in the South Atlantic. And that yielded really useful information about how modern fly-by-wire systems create some problems. So we now have some airlines deliberately encouraging their pilots to go off autopilot at high altitude cruise, just to get practise and 'feel' for the aircraft when it's in that always close to stall condition you get up high. Plus there's better training for what happens when fly-by-wire defaults to manual mode. And hopefully more thinking about how electronic cockpits can communicate with pilots when some of the data they're showing may be unreliable - giving the pilots a better chance to save an aircraft where the computers have given up in confusion and dropped everything back into the pilot's lap.
As a bonus here we get to learn more about ocean currents and environment - which all feeds back nicely into climate change research, which seems to be quite sketchy on ocean data.