I'm renting an RF 400mm 2.8 for my up coming brown bear trip in July. I know there will be alot of good used RF 400mm 2.8 lenses hit the market as people upgrade. I debating how much I really want the built in teleconverter or how much I'm willing to pay for it. Truth is I really do want it! I would prefer a simple reliable 1.4 switchable teleconverter versus a complex 1.4 & 2.0 setup. Will have to see what they release.
The price is going to be interesting. The current Canon RF 400mm 2.8 is $13,400 new. The Sony 400mm 2.8 is $13,000. The Nikon 400mm with a built in teleconverter was $14,700, but is now $12,700 (B&H prices). I would be really annoyed if I bought the Nikon lens for $14,700 and it just devalued $2,000.
I prefer zooms for wildlife, but 400mm at the wider apertures is really nice for low light and bokeh. I was seriously considering the 300-600mm f4-5.6, but I'm not interested in a fixed 5.6 for the projected price. The original article mentioned that the Canon EF 200-400mm f4 with 1.4 teleconverter didn't sell well. That surprised me for 2013. I considered this lens, but not in love with the 8lb wt for the price. Maybe Canon is having second thoughts about the 300-600mm. I have the 100-500mm f4.5-7; it's actually a great lens. But not in love with the teleconverter situation, external zoom, or 7.1`on the long end. An updated f4.0-5.6 100-500mm or a 180/200-600mm would probably sell better then a 300-600mm.
I'm sure a 500mm 5.6 prime for $4000, similar to Nikon's small affordable prime lens, would make a lot of people happy. A 700mm f6.3 would also be great.