A 500 f/5.6 would be substantially heavier than your 400mm f/5.6. The diameter would have to be 25 percent wider, 56% more area. The big element would probably be 95 percent heavier (because it'd be 25 percent wider, taller, and thicker). Other optical elements would probably scale up. The metal tube and so forth would go up either 25% or 56 percent (or something in between) because that will scale with circumference and possibly focal length.My EF 400/5.6L is my oldest lens and I've had since film days. Probably my best investment. My main uses are sports (windsurfing) and birds, and like someone already mentioned, if I had a 100-400mm, I would use it at 400mm 95% of the time. Sure the latest zooms may well be sharper, but this lens is 30 years old.
I would definitely be in the market for a RF 500/5.6 that is sharp wide open. Not interested in zooms or faster (heavier, more expensive) primes.