@MARC Rider The answer to how many can't fly is...complicated. A lot of people fly when not advised to do so - there usually isn't anyone checking flight fitness. Airlines have the right to demand a flight clearance from a doctor, but very rarely do so.
That said, anyone who has a cardiac issue, sickle cell, a history of stroke, a history of DVT or any other condition that requires supplemental oxygen or impairs oxygen intake such as COPD likely would not be cleared to fly due to the effects of pressurization on oxygen levels and the body. Same thing for anyone suffering from any sort of suppressed immune system - the effects of being in a small area for an extended period of time, re-breathing shared air are a recipe for contagion.
This never was an issue for the most part until recently, as most folks who suffered from these sorts of conditions were either A. Elderly or B. Disabled and rarely traveled. However with COVID-19 causing long COVID which affects both heart & lungs, particularly the uptake of oxygen as well as causing immune system dysfunction in a subset of folks....we now are seeing a much larger group of working age adults who should not be flying. We have data at present that indicates around 30% of COVID suffers go on to develop some form of long COVID, and the same data shows that about 5% to 10% of the overall population has disabling symptoms that interfere with standard activities.
If I had to guess at a number overall? I'd say somewhere between 15% and 25% of the population of the USA likely should not be flying. It could potentially be as high as 30% or even 35%, but a chunk of those aren't aware they shouldn't be flying. If we add in those who cannot drive, that likely jumps to as high as 40-50% of the population by the time we factor in vision loss, medical conditions prohibiting driving such as epilepsy, and neurodivergency conditions such as autism or ADHD that can make driving difficult.
Unfortunately as has long been the case, most disabled folks are expected to do without or pay their own way. Public transit for disabled in America is a headache, with every single little burg, county and area doing their own version of the same federally funded programs - most of which are only open to senior citizens or the very disabled who are nearly housebound.