I'm not sure I see how standby for an earlier flight blocks passengers with flexible tickets from getting on an earlier flight. A standby that is confirmed just before the door closes (as is usually the case in such situations) is behind anyone with a right to book a seat on that flight, which would include the passenger with a paid ticket. At some point the flight must be closed for boarding (e.g., BA's conformance times at T5), and at that point there is no possibility of any additional passengers changing tickets to travel on the flight in question.