You've hit on one of the fundamental cultural conflicts in NASA. Historically, they began as an R&D outfit, NACA. In that regard, they did some very important work on things like drag reduction, airfoil development, structures, safety, materials, and systems.

When the space mission was added and the agency changed its name to NASA, there was a conflict of institutional philosophy. Which were they, an organization dedicated to R&D or an organization dedicated to running space operations. The two missions aren't interchangeable.

Back in the mid 1980s, the Air Force had to issue guidelines to keep the engineers from "tweaking" every single space shot. Engineers love to change things. As a result, no two missions were ever really alike. The Air Force was getting ready to launch the GPS constellation. They couldn't afford for all of those Delta boosters to become engineers playthings, so they set up procedures to standardize the vehicles. From an operational perspective, imagine what it'd be like if every fighter and bomber to come out of the factory had been tweaked. The costs would've ballooned and maintenance would've been a nightmare.

This conflict of cultures heavily impacted the ISS development. Instead of working to build a functioning space station, it became an exercise to push the newest technology whenever possible. The result was a blown budget and schedule. The ISS is an over-engineered system where virtually every part had to be custom made. To me, this is reflective of poor internal discipline. NASA spent close to $10 billion on design studies before cutting the first piece of metal. The whole project was supposed to cost $8 to $12 billion.

Contrast this with how the Russians have ran their space program. Their SL-4 Soyuz booster is a direct descendant of the SL-1 used to launch Sputnik and the SL-3 used to launch Gargarin. Add an upper stage to the Soyuz and you get the SL-6 Molynia. Following some early teething problems, the SL-4/6 has some 2000 missions with a very high reliability rate. With few changes over all these years, there are no high R&D or tooling costs to amortize, resulting in a cost of about $1000 per pound to LEO. The Russian philosophy has been "if it ain't broke, don't fix it." They have implemented incremental improvements in their systems over time but they maintained discipline in their engineering. The result is a family of boosters that are economical, reliable, and capable of meeting most any need.

As for your proposed capsule idea, it's good. I'm not certain what the requirements are to consider a booster "man rated" but that would have to be factored into the equation. Building a single capsule capable of being integrated into a wide range of boosters could be a real engineering challenge but I think it's possible. It might require a custom integration adapter for each booster type to handle the issues of electrical and physical interfaces. That shouldn't prove too difficult. Weight and balance might be a tougher issue but that should also be solvable.