We very occasionally use the bus- dh is a wheelchair user (powerchair). I fold up the buggy, carry youngest dc. When he was small, I used a sling. A handful of times, the driver has had to clear whole families sitting with a toddler in the wheelchair space
. Dh is quite easy going and will suggest they can actually fit the buggy in at his feet- but tbh it's their risk- if the bus stopped suddenly and his chair slid... some people seem incapable of folding their own pushchair and make a massive fuss about it. They ignore dh- as if he doesn't exist- if he says anything to them. I get on behind him, carrying our buggy ready folded- they ignore me, too. 
When our dc1 was small, the low floor buses were only just being introduced (14yrs ago), so I used to put him in the sling for the bus and fold it, then put him back in the buggy when we'd got off. Or the driver would hold him, or someone else would- I never had to ask actually, because I was quite capable of holding him and folding it up/carrying it myself, but people used to offer to help us. I've done buses on crutches with 3 small children with me, folded the buggy and carried it on/off. It's not that hard to do, I really don't understand- it's really not the hardest thing you'd encounter in life- how do people manage with other stuff? How did they assemble the buggy together in the first place, if they don't know how to fold it? Where do they store these unfoldable buggies at home? I have a silvercross sleepover pram- it folds easily, two sections to it- chassis and seat- I don't use it for the bus of course because it's huge, but it folds, it is currently folded and occupies a smaller footprint folded than when it's not. And if some insane reason it was absolutely necessary to take it on a bus, I'd be confident in my ability to fold it and carry it, with small child and bags, because I'd have to. And because it's actually not that hard to do.
Dh sometimes offers to swap if people say stupid things to him. We'd both sooner our only concern about catching the bus was how to fold up the buggy. I'm afraid I lump together people who can't work out how to move the buggy out the bus wheelchair space with the people who park over lowered curbs... or shield their children from dh in shops (maybe it's catching... or maybe he'll run them over...), or the strangers who pat dh on the arm and speak condescendingly... or who do a double take when they realise we're with him, as if being unable to walk far means you're also unable to be married and have children...
There's a lot of ignorance about disability out there. Also a lack of common sense!