Just picking up on the last sentence you write
If travelling on a through ticket then an IATA airline is obliged to look after you and honour the onward flight booking even though you may be travelling on a restricted ticket.
Totally understand if under MCT that an airline on a through ticket has to take responsibility. But if a plane lands on time and MCT allowances and people then get stuck in security for example – can the airline deny responsibility should a passenger not make connection. From what I understand they could.