[HeyIts007] ... maximum effort would have been best spent early and not over decades
Initially no one had any idea it had flown for hours to nowhere; it's pretty difficult to see how anyone could have done more sooner.
Admittedly too much time was wasted in the wrong area because the Malaysians didn't believe it hadn't just crashed on route to China.
It's clearly a slow process now to map the sea floor and look systematically in areas thought promising. They might get lucky, they might not.
You seem to be criticising the search because the outcome wasn't to your liking. That doesn't mean it was wrong.