What's really clever is that at the scale that GA is deployed, it's really really hard for Google to willy-nilly break API just to get around this because a lot of webmasters will simply not bother updating their scripts, and if Google forcefully pushes a breaking change, people might stop using GA, or worse, they get an avalanche of bad PR for breaking half the web.
My understanding is they are veeery careful around rolling out changes and deprecations. The cleverness is that there's a huge asymmetry in how fast Firefox can globally deploy updated shims vs how fast Google can change GA API.
