I think we need to think two steps ahead here. Say everyone starts using ad blockers and disables flash. Then what? Websites still need to make money.
In this case Brave is betting that websites will want to ask for money from their users, but that each website maintaining their own subscription service is not viable. Maybe large websites like WSJ can get people to subscribe, but most probably cannot. Do you want to be subscribed to every website you ever read a single article on?
I think Brave in this case is positioning themselves to be the middle man. They want to become an essential layer that sits on top of the ISP. If their vision of the future turns out to be accurate, being the first mover here will be very profitable.