Regarding overbooking: Mr. Businessman can book three consecutive flights from A - B on the same day being unsure of when his meeting will finish. He only takes one of those flights and indeed only has to pay for one flight. If airlines only booked to aircaft capacity most flights would leave with empty seats. We are not talking one or two but as an example AMS - LHR the number would be nearer 25 - 30.
Until it becomes normal practice that all airlines require all booked seats to be paid for then overbooking will remain a frustrating fact of life for passengers and airline staff alike. For an airline to unilaterally go down this road would be akin to commercial suicide.
I believe hotels have similar problems.
Theatres don't, if you book a seat you pay for it!
Overbooking is a remarkably precise art but unfortunately not perfect and some will always face the appalling situation of either being downgraded or denied boarding.