That's exactly what I do. First of all, it's annoying that I have to go and choose each one to detach it. Second, I usually invoke tmux directly with mosh or ssh -t, so detaching also disconnects them, which I don't really want.

I basically want an untouched session to go into a screensaver mode where it no longer controls the terminal size.

It's certainly not the end of the world to detach them, but I'd rather not. And detaching before I leave a session is just busywork.