"If it completely can't get a font, presumably it'll use the same fallback rules it already uses when you call for a font that isn't there."

The question is what defines "can't get a font completely". Is it a timeout? How long is this timeout? If I spend time and resources to optimize the loading time of my site and the JS script times out getting a font, isn't that lost time for me?

This all feels like a workaround for license issues as opposed to a true solution. Type foundries should let web developers purchase fonts that can be published on the web.