I'm sorry, but this isn't unfair. You were willing to wait 5 years when you ordered the aircraft and nothing changed. The player who picked up the BK slots was going to end up getting aircraft further out than 5 years and decided not to put an order in. Only when the timing changed to <5 years did it make sense for him to put an order in.
Also, you are complaining about production slots for the most popular aircraft in the game. If waiting 5 years is an issue, fly something else. You could have also rolled the dice like the player in question and risked getting your aircraft in 10 years instead of 5, but you chose not to. There are no solutions in business, only trade-offs. In this instance, the player's risk paid off, but most of the time it won't.