Someone mentioned about needing to read the documentation of vim to learn how to exit the program. I personally think exactly the same applies here, and am a bit surprised at the depth of discussion over this.
When I first was new to cli programs, the only "standard" way to exit a program I found across the board was CTRL+D. I have never even thought that psql, even though different, was odd in its choice of how to exit the program. And their choices for why \q is used in a SQL cli program have very good reasons as has been discussed.
So what if that stack overflow is the most hit on psql questions? How many of you have learned what you need within 5 seconds on google? I don't think this is a significant stumbling block for users adopting postgres. psql is amazing and for anyone who likes cli programs, they will be fine.
FWIW I am +1 in favor of not overcomplicating the special psql commands and keep to the "\" standard.