I shoot wildlife and larger birds (eagles, etc). I have the R5m2, R6m2, 100-500, 70-200 2.8z, & 1.4 teleconverter. I really like the 100-500 for size, wt, portability, and IQ (image quality), but not in love with the external zoom, teleconverter situation, or 7.1 at 500mm. I want to move up to a semi pro /pro lens, but I don't want a prime. I like the versatility of a zoom for composition and if an animal moves closer. At the moment, there is no RF L series long glass zoom upgrade!
The Canon 10k 100-300 2.8 is a great lens, but for me this lens would have to live with a 2x teleconverter on it. It doesn't make sense to me to send that much $ and then degrade the native lens IQ with a 2x teleconverter. Just me and I don't own the lens or 2x.
There is another "rumor" of a 150-600mm 5.6L lens to compete with Sigma, Nikon, & Sony. This would be more versatile then a 300-600 and also a 5.6, but don't know specifics. The 100-500 L was released in 2020; is this a replacement? The Sigma 300-600 f4 is a 6K 9lb beast with 3rd party limitations, not for me.
A Canon 300-600 5.6 would cover 75% of my shooting. I'm good with 5.6 at 500 to 600, but I don't think 5.6 is acceptable for a high end / expensive lens at 300 to 400mm. Really needs to be a variable aperture 4.0-5.6 OR a fixed f4.8 (splitting the difference), especially if it's replacing the EF 200-400 F4 with teleconverter and/or a companion to the 100-300 2.8. The 200-400 is 280-560 5.6 with the teleconverter switched on ( basically a 300-600 5.6), BUT you still have the f4 option. I don't think they will release a fixed f4, because it will compete with their 600 f4 prime. Will have to wait ( hopefully not too long) and see what they release & the price!!! Also, where is the variable / switchable teleconverter, there is a demand for this option.