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.
I would also bet that no FD lens that Canon ever designed came even remotely close to producing the kind of IQ that a modern Mark II supertele produces. An f/5.6 aperture at 600mm is also quite a bit larger than f/2.8 at 200mm (102mm vs. 71mm), so from the get go we are talking about a particularly non-trivial front element.
Zooms require compromise, and the greater the zoom ratio, the greater the compromise (especially when the wide end varies so much, in terms of AoV, from the long end.) The 70-200 has a 2.77x AoV factor (34.4°/12.4°), where as a 150-600 would have a 4.32x AoV factor (17.8°/4.13°). They aren't similar enough to be compared, and even though the patent is for an f/5.6, I would be willing to bet hard money that a 300-600mm focal range (which has a mere 2x AoV factor (8.25°/4.13°) is more amicable to modern Mark II IQ than a 150-600mm focal range.
It's unfortunate that third party manufacturers seem to sell more lenses with a big zoom range than high quality primes, or good zooms with a short zoom range. I guess we need more people birding. All it would take is a good 600f5.6 lens and most of the large supertelephoto lenses would become practically obsolete (or at least redundant), but it sounds like there will never be a big enough market for that without company pride on the line.