These birders all envy magnification and IQ of 300/2.8 or 500/4.0 but not only are unwilling to pay this kind of money - more importantly these lens-designs are to heavy for them to take them into the field.
I'm finding this whole thread a little confusing.
As a non-technical person, it once seemed logical to me that telephoto EF-S lenses could be made longer and lighter, but after reading many discussions of this on this forum (other threads) I gather that is not the case.
So, I'm wondering what is the point here?
Yes, physics is a real pain in the rear. But, it seems pretty clear to me that regardless of brand, once you get beyond 400mm or below f 5.6 the price skyrockets. I'm guessing that it might be possible to produce a 500mm f5.6 that's close to affordable and it would be nice if that were to happen. But, that's probably pushing the limit of what could be available with today's technology and manufacturing processes.
If it were practical to produce the lenses you suggest and make them affordable, someone would most likely be doing that. But, even Sigma's lenses get real pricey at the long end. The most "affordable" option is the "bigma" but it's more than the Canon 100-400 "L" and isn't nearly the lens. Their 500mm f4.5 is $5,000 and the prices go up from there.
Semi-superfluous comment No. 1: I think you're selingl the 100-400 "L" short. I think it's a better lens than you give it credit for being.
Semi-superfluous comment No. 2: Anyone with more knowledge care to comment on catadioptric (mirror) lenses. Both Canon and Nikon abandoned the lenses. While the the fixed aperture and donut-shaped bokeh aren't ideal, I don't think of them as deal-killers. Do they have sharpness issues and are these insurmountable?