Beyond-the-horizon radar imaging is hard. And civilian aviation rarely has any need for it.

Moreover, such a radar would be a military radar. Were Malaysia to use one on this route it would not be long before the Vietnamese military jammed it.

A more realistic approach would be to have "transponders" that periodically sent the plane's position to satellites, similar to the way a GPS-equipped PLB sends location to satellites. There are problems of cost and where to find the radio spectrum (hint: anyone with good spectrum for satellites will not surrender it easily). Couple that with the need for international agreement ... I suspect a lot of countries are willing to accept a mystery disappearance every 50 years rather than spend the money.

This is a case where absence-of-evidence eventually becomes evidence-of-absence: the plane probably didn't crash into the sea where they're looking. The worst case for SAR is if the engines kept running after whatever inhibited the transponder, and the pilots were able to maintain powered flight.