I would like any telephoto lens to be as light as possible. I owned the FD 400mm 2.8 and at over 6Kg with a camera body it was very difficult to use. My EF 500mm ii is much more manageable at 3.97Kg on a R6ii with adapter. Currently the RF 600mm is 3.76 with a R6ii, how much heavier is acceptable for the 'benefit' of two built in extenders. I would prefer the extenders in my backpack for when I need them rather than supporting their weight in the my left hand when I don't.
A 600-1200 f/4-f/8 zoom would be a nice twist and much more useful that dual TCs.
and weigh a large number of pounds... and it would cost $30,000
For what dental care costs here, ours could easily afford them.I met a retired dentist (how cliche is that) in Japan who owned both this Nikon and the EF 1200/5.6.
A 1x-1.4x switchable TC is NOT just a matter of 'adapting it into a separate teleconverter'. A teleconverter mounted behind a lens, with the optics 'flipped out of the way' would be an empty tube between the camera and the lens...that's called an extension tube, and what it does is reduce the minimum focus distance at the cost of losing the ability to focus the lens on distant subjects. Probably not what you would want with teleconverter on a telephoto lens, y'know?As for a switchable teleconverter; I would prefer a completely separate unit. That way it can be used on other lenses and when it's needed. A built in teleconverter adds weight, length, and cost to the lens. I only need 1.4, not a fan of a 2.0 teleconverters ( my personal opinion). They have a working switchable teleconverter from the EF 200-400mm, that has shown reliability for years. Why haven't they adapted this into a separate teleconverter; there is no doubt in my mind it would sell. I would buy it.

Any new 600 f/4 is going to be expensive, but if the zoom is built as a variable TC (i.e. magnifier), rather than the more elaborate approach used in most zooms, the size hit might not be that significant. The current approach to 800 and 1200 lenses takes the magnifier approach and making it variable is not impossible. I have an interesting Vivitar 2x macro TC (in FD mount) that is a combo of a 2x TC and a variable extension tube rolled into one. With some of Canon's recent exploits, it is not unreasonable to expect that they might take a new approach. Not much doubt that any sensible way to combine several Super Telephotos into one would be a big cost saver for Canon and might even reduce the price a bit thanks to increased production volume of the single design.and weigh a large number of pounds