Waaaaay back in the dawn of time, the chat window operated as you suggest. It also had only one channel. And it had timestamps, which did not work. When chat was revamped, the timestamps were removed and the window closed automatically on reload. I know this has been brought up before, but I can't remember whether the devs said (1) they're working on it, (2) they'll get to it but chat is low priority, or (3) that's just the way it has to be, sorry.
For the record, I wholeheartedly agree with Brovnik. Player interaction is what puts the (second) M in MMORPG. Out-of-game alternatives like Discord just aren't the same as interacting in-game. (Especially for people like me with ancient cr@ppy technology that fights with Discord and loses.)