The question has always been about money. Building a new set of terminals is likely to cost billions, which means all IAD-based fliers will be paying higher fares to pay it off for many, many years once it happens.
No, they won't. Fares are based on what the airline thinks they can get away with.
I agree with almost everyone that the facility is a dump. The big 'however' is, whether people book away from the facility itself or the location or airline. You'd have to be blindly loyal or a fool to book a flight out of IAD if you're based in the district itself and you can fly to where you have to go out of DCA. I'd rather connect in CLE/ORD/DEN to DCA than fly to IAD. Perhaps that would change with the metro line, at least you wouldn't have to deal with the awful traffic.
That is why IAD is left with some long haul and hub-hub NB's, the expanding express operation and the international traffic that can't use DCA.
At some point the new building will become a necessity rather than an option given the actual physical structure integrity, but I highly doubt the investment will pay off in any meaningful, non-utilitarian sense.