Mandated by the ADA, all forms of public transportation should allow for equal access, including participation with dignity.
Initially, it was thought that it was impossible for buses to be wheelchair accessible, until manufacturers rethought the design making buses better, easier and more comfortable for all such as elderly, those carrying packages, mothers with baby strollers, etc.
Manufacturers such as Boeing and Airbus will design and build airplanes that can accommodate roll-on/roll-off boarding and safe securement of a person using a wheelchair should the air carrier purchasing the airplane include that requirement. To say it is impossible to design or build is nothing more than the same excuse.
In fact, any aircraft bigger than an Airbus A319 or Boeing can currently accommodate roll-on/roll-off boarding and deplaning with little more than a change in attitude, change in billing system for the first row of first class an and inexpensive boarding ramp and already proven crash-tested securement straps that can attach into the tracks built into the airplane that the seats are currently mounted on which allow for adjustability of the seats forward and backward.