Bicycles are not an 80/20 solution, they're a 99% solution, with the appropriate kind of bicycle and accessories. You can haul all kinds of crap - construction equipment, children, appliances, etc. - with a bicycle.
There are many other kinds of bicycles that make them accessible to people with reduced mobility: tricycles, hand-cycles, electric assist. Further - active transportation also counts as exercise and physical therapy, even further reducing the number of people who end up with reduced mobility in the first place!
I don't really know what a startup could possibly do in this area that's not already being done by the dozens of active transportation advocacy organizations at every level of society.