I think the assumption here is that if you have multiple displays you would rarely need full screen apps in the first place. It's definitely a feature that work

I think the assumption here is that if you have multiple displays you would rarely need full screen apps in the first place. It's definitely a feature that works best on a laptop display. The best alternative I can think of is to full-screen all the windows on each display. So for example if you had 3 monitors with 3 Safari windows, one on each display, they would go full-screen on their respective display. (downside is it would be awkward to leave full screen since the controls for it reside on the menubar)