If a two stage liquid propellant rocket could be built wherein both stages could be checked out, refueled and used again in short order, that would be a major step.

The motor takes the biggest beating of all of the components, and may just be impossible to reuse without overhaul. On the other hand, the development of new alloys might make motors more durable. I know of no rocket motors that can be reused without complete overhaul. Perhaps overhaul of motor only, with lots of motors being overhauled at the same time, would work. Suppose that it took 30 days to get a motor ready for reuse. Then, with 30 motors, we could launch the vehicle once per day; with 60 motors, twice per day, etc.

In any case, throwaway rockets will never be cheap. Reusable rockets could cost less, but might cost even more, as in the case of the Space Shuttle. Only reusable vehicles offer the opportunity for large cost reductions.