I guess to answer the question, Miyata just made a good solid bike for the money. I have only owned one Miyata, but I've had it for 38 years. I had it for 27 years before I bought a newer bike because I never saw it as an old bike, it was just my bike. I never felt like it was the bike holding me back. It still gets ridden regularly. I have made some concessions to accommodate my aging body, like the stem and saddle, but it is mainly the same as when I bought it in 1984.
1984 Miyata 310