I think I can answer the second part of of this one, having recently spent time working on a bid to a certain governmental organisation dealing with aviation to replace their data network. Similar thing to AFTN, in that flight-safety critical data was intended to go over it.
TCP/IP and the internet ARE NOT RELIABLE enough. The network design proposed was ultra-reliable, in that it had multiple paths in case of failure, and the fall-back paths were of different technologies. Yes, the X25 network traffic was encapsulated in TCP, but then prioritised to ensure it reached its destination ahead of other traffic. This guarantees data delivery. Can't do that on the internet.