> Why was iOS's inclusion of ad blocking capabilities the turning point in what seemed to be an already-unstoppable movement towards a return to sanity for the web?
Because iOS was the last major platform on which true adblocking was (AFAIK) completely impossible until this week.
Desktop browsers have had adblocking for ages, and even non-rooted Android devices can block ads in webpages (root is required only to block ads in native apps).