The FD version was 150-600/5.6. So should this be. 300/420 at the WIDE end isn't wide enough. Sigma and Tamron both have super tele's with wider zoom ranges than 2x.
You have to figure there would have to be IQ compromises to support 150-600 though. In the film era, the difference would probably not have been noticeable. With constantly increasing sensor resolution these days, I'd rather have a 300-600 f/5.6 if it means the lens is sharper with better contrast.
The 70-200/2.8L IS II shows the folly of that thinking. Building an f/5.6 lens to be optically excellent is much easier than building an f/2.8 lens.