They actually have a way to show this. If you play around with the other buttons and modes they seem to use colored for on/connected, grey for on/disconnected, and grey with a line through it for radio off.
If you put your iPhone in "Airplane" mode, the wifi shows "OFF" (with a slash through it) vs toggling the wifi button just grays it out and shows "not connected"
Seems fine for me /shrug