I think the lower cost will be the main attraction for a 600/5.6 lens. It has to be less then the $12.4k 600/4.
In any case, f/5.6 seriously limits use in low light and with teleconverters. Pros would prefer the f/4 and most everyone else would be fine with a 150-600/6.3 or 200-500/5.6.
I see there is a new 70-200/4 IS EF lens.
From Nikon Rumors, "Nikon officially announced the development of the previously rumored AF-S NIKKOR 500mm f/5.6E PF ED VR lens."
Smart announcing the 500mm before a 600mm f/5.6 PF/DO lens. They can price the 500mm rather decently, then price gouge the 600mm f/5.6 a year later. Although the 500mm will be a full-frame (FX) lens it will be targeted to the crop (DX) D500 crowd while the pricey 600mm will be marketed to the full-frame folks.
Canon updated the 70-200/2.8 IS as well. But it appears all the 2.8 lens got was a new paint job. At least the price didn't get crazy high like the Nikkor 70-200/2.8E.
The Canon 600/4 DO has been rumored and in development forever. Obviously an f/5.6 should be cheaper to build. Technologically a 500/5.6 should be easier than a 600/5.6, but it would be more economical to produce both together using similar designs and some shared components. A 600/5.6 would be far more useful since there is already the 200-500/5.6.
Hopefully the rumored price for the Nikkor 500/5.6 PF is just that; a rumor. Realistically the mid $3k mark should be the final price. Say... $3496. Then the 600/5.6 can be $4396. Again the IQ has to be there for those prices. Better than the 200-500/5.6 level but a bit behind the 500/4 & 600/4 super-primes.
Pics of the 500/5.6 PF at the Nikon World Cup party. It looks tiny for a 500mm lens. I really hope the price is under the $4k mark instead, of the rumored $4300 price.
The 600/4 IS III is shipping next week. I'd probably buy if it were out sooner and a DO lens. That 18' length is a bit much for my bags.
Canon 300/2.8 and 500/4 to receive an update middle of 2019. Probably the same types of updates as the new versions of the 400/2.8 and 600/4 lenses. No word on 200/2 or 800/5.6 updates. Perhaps just before Tokyo 2020.
Canon displayed a mock-up/prototype 600/4 DO a few years ago. I wonder what happened. I think Canon is missing the boat, as far as sales go; for the, "I want a big telephoto lens but will NEVER, EVER, NOT in 2352416137 years be able to afford a $9K+ lens" semi-pro/well-off-amateur folks.
The Nikkor 500/5.6 PF looks pretty good both in IQ and size/weight. Two 500mm Nikkor lenses available now that aren't "Super Telephoto" in cost. 200-500/5.6 @ $1400 and 500/5.6PF @ $3600. Over 400mm in the Canon world is $9k for the 500/4. Probably sharper and obviously a stop faster, but still $9k. I think there is room in the Canon lineup between the 100-400/4.5-5.6 and the 500/4 lenses.
I would be very interested in a 600/5.6 PF Nikkor.
Probably, maybe in 2019. $4500-4700. 35-40% of the 600/4E cost. Everybody who is/was planning on buying a 600/4 probably has already bought the big boy. 600mm is a threshold people want to obtain but will never spend $12,300. I think people are holding off buying the 500/5.6 PF in the hopes of the 600/5.6 PF becoming available semi-soon.
What will happen first; 600/4 DO or 600/5.6 PF?