Bus drivers already have to assist handicapped passengers that are getting on and off the bus (at least for my metro area bus service), so someone will need to be in there anyways.

A better bus and stop design eliminates more of that. You can have call ahead for help if you need to. A busy bus route normal people will be glad to help the disabled as well so long as what they need to do isn't hard and doesn't take much time (which it shouldn't - see better design...). For the rest paratransit should take them because despite the higher costs disabled people if they need to much help are taking time from everyone else who wants to get someplace not wait while someone is slowly loaded onto the bus.