The DC-X was 9.1 tonnes empty and 19 tonnes full- meaning landing thrust was ~half of takeoff thrust. The Falcon 9 was 400 tonnes full and 26 tonnes empty, so takeoff thrust was >20x higher than landing thrust.

That's a huge engineering difference, roughly like the difference between a car and a helicopter. The Falcon 9 was also 4x taller, meaning 16x more force to correct a lean. A little burp would send the rocket right back up in the air.

Don't you mean the SpaceShuttle in the 80s? or Delta Clipper which didn't reach orbit?

Really, what SpaceX did was radically different from the tests in the 90s from the rockets, to the controls, to the reusability goals. Otherwise they wouldn't have built Grasshopper.

Now NewGlen is kinda a knockoff of Delta Clipper, but that's a different beast.

And physics, nothing prevents the goal beyond execution.