Considering the following:
RF 70 - 200 f/2.8 - cannot add TC
100 - 500 - can apply TC 2.0 or 1.4, but only starting at 300mm
That means with a 1.4 you get 420 - 700mm, and with a 2.0 you get 600 - 100mm.
With the 1.4 on I have a hole in my bag from 200 - 420mm, I have to take the TC off to get there.
With the 2.0 on, I'd have to swap to the 1.4 to get 500 - 600mm, or take it off entirely to get 200 - 420mm.
I could carry the EF 70 - 200 f/2.8 IS III instead of the RF, which allows stacking a 2.0 and a 1.4 TC (MK II models only) for 196 - 560 f/8, but now I have to carry 4 TC's and an EF converter to maximize my options. At that point, I could leave the 2.0 on the 100 - 500 and my only "hole" is 560 - 600mm, which basically isn't one....but now I'm operating with sub-optimal setups IQ-wise on both lenses. And while I use the EF 70 - 200 f/2.8 + stacked TC's + EF converter setup today, it's very long and heavy, and doesn't give anything the 100 - 500 + 1.4 wouldn't if the full range was available (actually would provide less).
If I'm willing to carry (and buy) 3 lenses, I could do an RF 70 - 200, an EF with the stacked teleconverters, and a 100 - 500 with the 2.0 on it. Or, an RF 70 - 200, and two 100 - 500's, where I'd use the one defaulted to no TC's from 200 - 500 and add the 1.4 when 420 - 700 is needed, and the 2nd one would have the 2.0 on it all the time and be for the situations where 700+ is needed.
Another option would be if Canon made like a 200 - 400mm f/4 zoom. I suppose one could carry the forthcoming 100 - 400mm for that, but it's slower than I would like, especially considering a 2.0x-extended 70 - 200 f/2.8 gives a constant f/5.6 wide open.
None of this would be necessary if the RF 70 - 200 f/2.8 took TC's as one would expect, and/or the 100 - 500 got its full range when on a TC.