The re-usable STS i.e. the shuttle, was never supposed to last as long as it has, hence why they're falling apart
The best replacement sytem they could go for would have been a
"space plane", similar in concept to the Space Ship One that won the X-prize, and is the basis for Virgin Galactic. But because this concept would have needed huge amounts of design, testing and experimentation (even though it would give the best long term benefit) they've opted for the short term "cheap" option of tried and tested rocket booster stacks.
I won't even mention the safety issues of strapping a crew in a tin can onto what is essentially a poorly controlled bomb...