Seriously? Somehow everybody is fine with the 100–400mm with 1.4x at f/8 but the same lens with a "1.25x" builtin extender that you don't even have to toggle on and off is somehow "not L"? Sheesh…
The only reason f/5.6 was the limit for so long was that DSLRs could not focus reliably much beyond that. But DPAF can focus down to f/11, and at the same time sensors and IS systems are better than ever. There's absolutely no reason to artificially limit engineers by imposing an arbitrary f/5.6 max aperture limit.
Arguing about all this is kind of senseless until we see the aperture transition through the zoom range and the weight and size of the lens.
We have no idea if it can do 400/5.6 but I fear it can not. In case it can and just jumps to 7.1 above 400mm and they even managed to have it the same weight as the 100-400, I say it's good.
But if not, then I would be happier with just an RF version of the 100-400 which is an excellent lens, and who knows, maybe the RF version could even be lighter which would be MUCH appreciated.
And no, I do not use a 1.4X with my 100-400. Shooting at max f/8 which is actually at least f/11 if you also want it to be sharp is usually not something you want.
I disagree with your idea of having a builtin extender which you don't have to toggle on and off -
it's an extender which you can't take off to get a faster lens or better IQ. Well, the whole concept of talking about a builtin extender is false I think. The only construct that can be referred to as a builtin extender is what we see on the 200-400. Otherwise we could talk about all the longer lenses as being the shorter ones with builtin extender. Silly. Just forget it.