Sometimes, it makes sense to retrofit a proven vehicle with newer technology to improve performance. A good example was when the Air Force reengined the KC-135 tanker planes back in the 1980s. The new engines were both more powerful (allowing heavier fuel loads)* and more fuel efficient. The end result was that 2 of the reengined planes were able to offload as much fuel as 3 of the old models. There's also a proposal to replace the 8 engines on the B-52H with 4 newer engines.

The Shuttles were designed in the 1970s and are a pain to maintain. The very high labor costs required to turn around a Shuttle for the next mission is one reason why the missions are so expensive. Even if we fix all of the known problems, I don't know how cost effective it would be to create those reusable flyback boosters. Technically, the idea is sound but I don't know if it's economically viable. Sooner or later, it simply isn't worth sinking a lot of money into a vehicle that's on its last legs.

*Those old KC-135s were very underpowered (The KC-135A had a max takeoff weight of almost 300,000 pounds and only 44,000 pounds of thrust - and only that much when you could use water augmentation during takeoff). I went to Squadron Officer School back in 1990 with a guy who was a copilot on one of the old models. He said that even when taking off from a 12,000 foot runway, they wouldn't even be fast enough to rotate until less than 1,000 feet from the end of the runway. He said that he saw his life flash before his eyes on many takeoffs, especially on hot days.