I think you are missing the point. All of that R&D createsd developing those “enormous & expensive” lenses can be leveraged across the entire product line. With that knowledge and experience, they can continue to expand the capabilities of lower priced options. We see this in camera bodies, cars , computers, etc. Where do you think the lens tech for the RF 600/800 originated - two generations of $7k EF 400 DO lenses. The original discussion was could a 200-500 be sharper and the answer is probably yes based on Canon’s capabilities. I think it is more a question if that type lens fits in their product strategy.
None of us know if it can/can’t be done. Let’s all just enjoy a little optimism. I think this type zoom is coming, but it will probably be the expensive version. The 300, 500, and 200-400 did not get the EF III weight loss / update and my bet is they will be the first RF big whites. I know a RF300 patent has been mentioned on CR.
Btw - I paid $7,500 for a 200-400 Canon factory refurb about 8 months after launc, not $12k. Sold in the move to R/RF.