The bus is part of a system, as you said, but it's only the visible entry point and when the system is designed around handling individual fares, flat rates, ch

The bus is part of a system, as you said, but it's only the visible entry point and when the system is designed around handling individual fares, flat rates, children, free fares for students and the elderly, as well as making it fair and equitable (and relying very much on fares for funding), then you have a bus which must work with that system. Some cities rely on conductors or a vast fleet of ticket inspectors, but London's public transport system might be the biggest and most extensive in the world, and some things don't work well at that scale.