> So what exactly does Plausible do to avoid being blocked in the same manner?

It seems they serve plausible from the same server as the origin website (first party), probably under an unique name, which means you need to block the script manually on every domain instead of simply blacklisting `plausible.io` (for example).

> What prevents GA from doing the same?

That's a good question. Probably ease-of-use (no need to explain how to host it, easier updates) and the lack of need to do so. YouTube, for example, could also make it _far_ harder to block ads by simply sending a single video stream, but they don't for some reason.