There isn't going to be that much appetite for this just so that people can use PostgreSQL without reading our documentation: or the output of typing "help" at the psql prompt that says "type \q to quit".
For example, while I think "quit" is a good idea, it'd be a shame if it caused people to never find out about \ commands.
I suppose the only bad thing about introducing "exit", which itself seems like the more common choice for program stopping command, is that someone might end up inferring that "\e" should work to exit the program instead of opening an editor...