Miaow the posters that people are getting frustrated at are those that do the "I have a big buggy, I am there first, and I have just as much a right to the space as a wheelchair user" kind of thing.
As long as the person with the buggy is physically able, there is really no reason they cannot fold up the buggy and move for a wheelchair user. The problem seems to be that many (note I said MANY, not ALL) people with buggies feel entitled to that spot and refuse to cooperate, and the bus drivers refuse to intervene.
When DS1 was a baby, I quite happily folded up the pushchair on the bus. Now that he's 7yo and in a wheelchair, I expect other people with pushchairs that are physically able to do the right thing and fold their pushchairs up and move to another seat. Sadly, many are simply not so inclined. It's a lack of common courtesy and consideration (not to mention a complete and utter disregard for rules). It's very much the whole "rules don't apply to me" attitude.
And those that are whinging that they can't hold the baby and their shopping and fold up the chair simply need to grow up. Practice holding the baby and folding the chair if need be (I did this in my living room before even venturing near the bus as I wanted to make sure I could fold the pushchair one handed). Hand the baby to another passenger for a few minutes (always had a number of people happy to hold DS1 in this instance). Ask for help folding if you need to. And limit how much shopping you are carrying or modify how you are carrying things (sling, backpack, frontpack for baby, whatever). In most instances, it can be done with a bit of effort.