But you are underestimating the challenge here. There is a difference between the landing and getting an orbital rocket first stage into a position to even attempt a landing.
The actual landing with propulsion is actually the easier challenge if you have an engine that can hover.
However SpaceX had to figure out how to get do supersonic retro-propulsion and basically making a heat-shield out of fire to protect the rocket on its way down, then navigate a terrible air-frame at high speeds and then do a never before tried hover-slam maneuver with an overpowered rocket engine.
That is very, very different from a DC-X that just goes up a bit comes straight down and then can do a slow landing.
I know some of the old space companies did some studies, you can also find a very long detailed post by Tori Bruno on the topic. The problem is, these studies and opinions are no necessary correct and in fact we know that they were very much incorrect. The assumptions both technical and economical these companies were making were simply not correct, at least not with the right engineering and investment behind it.
The study that you are talking about was from the 2010ish time right, not the 90s or something like that?
Arianespace/ESA totally fucked up with Ariane 5, instead of designing a booster that can dominate the launch market they designed a very niche booster and were forced to relay on Soyuz and couldn't beat Proton in many markets.
Partly this was because of Hermes and as they design Hermes it became bigger and bigger and so did Ariane 5. And then Hermes was canceled and Ariane 5 remained oversized.