It's really easy to serve fallback images to browsers that don't support AVIF, either client-side using the <picture> tag or server-side via the Accept header. Which mostly eliminates the concern from the article, since you don't have to drop support for any customers.

It kind of makes me wonder if anyone has made a build system / framework that serves nested CSS to modern browsers, and falls back to a preprocessed CSS file that removes all the nesting for older browsers.