What's the elevator pitch if I already know Screen and I can just open multiple windows of the terminal emulator?

No pitch - just use screen, most people use tmux in exactly the same way - open a few splits and switch between them with keyboard shortcuts.

I switched from screen to tmux due to rumor about the screen code base. Maybe not a good reason. But I don’t regret it, tmux works well.

Just an honest opinion of someone who didn’t have skin in the game. Not sure if it helps.