Agreed. Not necessarily 5.6 at 400, but I'll wager that when you have an established and successful multi-generation 100-400mm design that gives 5.6 at 400, all the designer basically (simplistically) needs to do it to make a telescoping tube a little longer and pull the objective lens a little father forward to get to 500mm. Honestly, if we took apart a 100-400 and just held the front element a little father forward, it would presumably image as a 500.
Granted, lens design has subtleties, and there would presumably be issues introduced (possible mechanical getting that telescope distance - I'd cringe at a three-part telescope). But it's hard to imagine that the change would require reducing the aperture at 400mm. That's the last solution the designer would be allowed by the boss to revert to.
As an aside, one hopes that the RF mount enables other benefits in image quality and product configuration. I now assume that the magnificent RF 70-200 that won me over to Canon late last year was essentially the 100-400 adapted to those specs.