X.400 was actually pretty great and was definitely better designed than the disaster of SMTP (I spent my youth in the email deliverability industry and know all

X.400 was actually pretty great and was definitely better designed than the disaster of SMTP (I spent my youth in the email deliverability industry and know all the ingredients of that sausage), but it wasn't a network transport protocol. There is no way that the Internet could have been built on the OSI transport and routing implementations. They were not good. Nothing has stood up to as much scrutiny and scale as TCP/IP. The simplicity is definitely an important part of it. Protocols that are used by trillions of devices need to be simple, and the equipment used to switch and route their traffic have to defer as much handling of state to the clients as possible. The more state that routers have to maintain, the less reliable they will be. TCP/IP accomplished that and that's why it was better.