I'm sure you guys would be just as annoyed to be sitting at departure time waiting for late passengers, just the type like you who arrogantly think they know everything and can make it to the gate just on time.
Just on time is not good enough. In all other modes of transport trains or buses, if you aren't there before the departure time then you'll likely not make the service. Same should and is looking like will apply in this new system, you can't turn up just on time and hope for the best.
This policy is also likely to have been introduced based on cost. The ground handling company at LGW is Menzies Aviation who have recently introduced an enormous soulless, windowless and fully automated check in area at Gatwick. This new policy seems more of a drive to reduce further staff costs. There is a lot of labour involved in escorting late/denied/offloaded passengers through arrivals, immigration and reclaim back to landside and to rebook their flights.