What made you expect anything different? Obviously the refresh of the 80-400 4.5-5.6 is going to have a similar aperture range. Otherwise it'd be a whole new lens. And if it were constant f4, it'd be more like a super-successor to the 200-400, which is in a completely different league altogether (at least in terms of price and weight). I'll concede the new price is hard to stomach, but it's still many thousands of dollars less than the next & last tier, of the 200-400 and the super-telephoto primes.
In any case, the quality with the 2x teleconverter is going to be marginal. The new 80-400 is impressive at 400 even wide open, but it's not working miracles. The 1.4x or maybe 1.7x are much better options (and this is true in use on almost all lenses) w.r.t. balancing reach with image quality.
FWIW, I've used the 1.4x myself on this lens and there's no obvious image quality degradation (or autofocus impact). Though I haven't pixel-peeped.