I can se an idea of calculating if it would have been theoretically possible to reach an airport, just for 'fun'.
IRL, when all both engines quit you have to make a rather quick desition that you believe will make you stay alive. @ low altitude there´s usually not so many options to choose from, and if you did make the choise and then another option comes visible you would have to have good reason to change the landing spot.
I´d say the crew did a perfect job, and I dont think that Sully wonder if (or should wonder about) he could have reached an airport safe. Remember that 'almost reaching' an airport probably would give a very tragic outcome. Even reaching an airport but not lined up with the runway could be bad.
I fly both fixed wing and helis, I work as a military heli pilot. @ flight school and for annual training a common way of learning the pilot a lesson is to cut the helis engines close enough for a place not perfect but survivable and then there would be a decoy (the perfect field for an autorotation) but that place is just beyond reachable range. If you swallow the bait you find yourself short of altitude over a big forrest or so about 2-300m short of the decoy. Lesson learned is to choose a spot 'good enough' and stick with the plan.
Anyone remember the SAS 'gottröra' MD81 accident/emergeny landing the 27th december 1991 ? Ice on both wing came loose and qiut both eninges at low altitide (about 4000ft if my memory still works). They where IMC and I think cloudbase was between 1000-2000 feet or so.
The captain got some kind of 'incapatity' and didnt do the work 100%, but another off duty MD81 captain rushed forward and helped the crew with the emergency prodedures.