Users would still be able to block Google and Facebook ads easily but would help some users in the meantime. Though as you noted they could simply allow content blockers to be able to block ips in an update. No need to wait to iOS 10 since it wouldn't be a new touted featured but rather a bug fix like they do all the time when people circumvent jailbreaking. So Apple could easily add that fix in version 9.1, 9.2 or anytime they like. Most likely they will wait to it becomes more widespread since dns changes aren't something the average person who manages their own site can update easily.