It'd break if you wanted to do the dumbest thing and just not stream the windows users aren't looking at; but lowering the stream resolution on the fly could work but now involves more complexity (on both sides to communicate when to adjust the resolution) and because it's not handled entirely on-device breaks the illusion of it being invisible.

I can also imagine it having some weird privacy implications; like Mac apps somehow monitoring this and tracking whether you're actually looking at them, etc.