If nothing else, NAT makes it harder to leave a walled garden for a small chat program between you and your friends/family/interest group. Without it, you could potentially all run the program and talk. With it, someone has to be technical enough to do port forwarding, someone has to be on a connectin without carrier-grade NAT so that port forwarding is an option, and/or someone has to pay with money/time to setup and run a server on a dedicated IP or upgrade their connection to a static IP.
The most easily accessible alternative is another company's closed system.
I suspect NAT wasn't involved in that developing as much as money was - IRC, email, Newsgroups were hard to profit from. Having a distinctive service which is different from your competitors so customers can be attracted, and which is closed so customers want to bring their friends and family in to talk to them and then can't leave without the whole group leaving, is bound to be more profitable for the winners than taking part in a federated system where only one user can join and talk to their contacts with relays to other servers.
2) NAT really screwed us all by breaking the end-to-end nature of the Internet.