Digital photography equipment depends on technology much more than the olden days. Last time we always try to dig old lenses from 2nd hand shop because optically they are often more superior than the later ones. Once they discontinued, our heart sinks. That is why there is a period of time where camera makers launched lenses where people are very reluctant to upgrade because besides having some minor features added, its not worth sacrificing for optical quality.
Now the situation is so different. DSLR made use of a lot of technology elements. So when the camera makers launched a new 1, it really make sense to upgrade because technical wise, they should outperform the older ones and there would be improvement they have made to correct the older replacement model or probably cater to future ones.