This was discussed quite a bit before the refreshed HDs came out. In short, Ram reused the existing cabs and beds in order to get the “4.5 gen” to market quickly using existing production lines. The 5th gen HD will be all-new, but it’s a couple of years out.I do wonder if FCA did not extend the cab because the HD trucks have the Mega Cab option?
But your point about the Mega Cab still being available is also true, and maybe that was FCA’s thinking as well.