> as primary means of transportation and arguably more importantly as a backup means of transportation

One bus route can't wear two hats. Faster, sparser routes are typically complemented by slow, meandering collector routes which provide the kind of backstop you describe. Moreover, elderly and disabled people can use paratransit [1], which exists precisely to serve people with mobility issues too severe for regular transit.

Anyway, I reject the notion of buses as a second-tier transit option reserved for poor and disabled people. The only way poor people ever get decent service is when they use the same infrastructure that affluent people do. A bus system that doesn't serve the middle class is a system that will quickly lose its funding and become inadequate for anyone to use.

[1]: https://en.wikipedia.org/wiki/Paratransit

Around 1/5 of the US population is elderly ~1/4 by 2050, add in moderately disabled people and this isn’t a small population we are talking about.

Paratransit is for a far smaller percentage of the population due to the significant expense.