I agree with that. But most airlines wouldn't know until the last minute. I no longer travel with my new job. But up until the end of February, practically every flight I took was booked solid. Airlines have a tendency to overbook.
As far as handicapped passengers go, they are boarded first. So it would be up to the airline to account for any extra room. The bottom line here is that it is a business decision (as it should be). Businesses have to weigh short term v. long term. As Anheuser-Busch just did. If an airline chooses to be a dick airline (like United, for example), then sure, they reap some minimal short term gain by selling an extra ticket. But in the long term, their reputation takes a hit.
That's all right... I have long been an advocate for the handicapped, because I have lived it... From being on a stick most of my life, to wheelchair and walkers. I am not ridiculous about it... I would not require a city to retrofit curb access for wheelchairs out of spite... But if they are redoing curbs anyway, it ain't no skin off their noses to install wheelchair accessible curbs...
Airlines are largely immune. Sure they have a wheelchair spot or two... but right now, I would literally have to travel with a wheelchair and a catheter because there is no way I could get to the bathroom, not to mention get in it. And right now, I am not really all that bad... getting around here with a wheeled walker.
And there is a whole lot of America that is legitimately 'otherwise enabled' My elderly mother has been left high and dry in Minneapolis and in Denver, because their assisted access in the airports totally dropped the ball. TWICE.
I know it's largely gratis. But it is part of doing business, especially because of many of the elderly. Handicapped should be no different within reason.