To paraphrase Lockheed, circa 1978, 'The SR-71 project was so far ahead of it's time in 1966, that the first aircraft off the assembly line was given the serial number "2001" '
NASA could today tack one of the SR-71C airframes up and run a world record beating flight. 54 years in later, and still nothing flying that can beat it, "officially".
The X wing helos were ahead of their time, but will one day fly again.
Actually, the only thing that killed the "X-Wing" project was funding. They work, their feasibility is not a issue. Yes, with today's technology (which was1983 Sikorsky's technology, as they were the largest manufacturer of composite products in the world in then, we made stuff for everybody, and nobody knew it was Sikorsky making them), they would be even higher performing an engines have become more powerful
Whether we really need such an aircraft is the issue. Two bladed stop in air rotary wing aircraft proved to be a failure. Balance issues.
The Whispercraft would have vibrated so badly, it was a concept that went no further than a render, and some paper models. Neat idea though. When a material can be made that has a high modulus, but can yet flex (an oxymoron), then you will see dragonfly like aircraft buzzing around.