I think we need to think two steps ahead here. Say everyone starts using ad blockers and disables flash. Then what?Websites die out.
We're only 30 years in to the web, and only 20 or so of those where advertising has been a workable system for funding an online business. In business model terms that's still very much in the "proof of concept" stage. Arguably the concept has been proven only to work when users have no way to block it. When they can block it and still view the media they often choose to do so, which means the business model fails.
This is true everywhere. The hayday of TV advertising was 40 years ago but now people have demonstrated a preference for subscription services that don't carry adverts (Netflix etc). Ad-funded print newspapers and magazines are dying because they don't offer something that many readers want, so their advertising model falls apart because they don't have the scale necessary to make it work. Radio seems to be bucking the trend somewhat as commercial radio is actually bouncing back, but that may be due to the costs falling dramatically.
Now it's online advertising lead businesses time for a reality check. 10 years ago online businesses could make a great deal of money by showing adverts to users who came for the content, but with the rise of social and the contraction in ad revenue they can't really continue. Maybe the answer is subscriptions (eg paywalls). Maybe it's micropayments (eg Brave). Or maybe, and this is what I'd bet on for the overwhelming majority of websites, it's just death. Everyone moves their content to Youtube and Facebook.