A pilot's view
timzsta
What you say is very true.
From the flight deck, when you have been given the loadsheet, normally after the last pax has boarded, you are minutes away from departure. It is, very literally, the last piece of the puzzle which must be signed to allow the aircraft to depart the stand and legally fly.
Anyone pitching up after this creates no end of problems if they were to be accepted fot the flight. Firstly the time issue. Is it right to delay hundreds of people purely because someone couldn't make it to check in on time? - FOR WHATEVER REASON!.
Missing a slot in Summer at some of the busier London airports can spell disaster for the airline. Potentially hours of delay, not only for that flight but also for all subsequent trips with that airframe. The likelihood that the operating crew, possibly on their 3rd or 4th sector that day, will now run out of legal duty hours and force the call out of a replacement crew. At worst it could cause the cancellation of a later flight on that line of flying - all because of 1 or 2 people who cannot see past their own personal needs.
Comments we have all seen on popular TV shows like '...but the aircraft is still on the stand just waiting......" show nothing but a complete, and sadly often deliberate, misunderstanding of even the basics of mounting a public transport operation (whether it be busses, trains or aircraft).
Believe me, we want to take ALL pax booked on any flight. We will all bend over backwards to accomodate them but, there comes a point beyond which we cannot reasonably go so, dear Pax, please don't ask us to.