Steem power holders suffer a 10% inflation. When adoption slows down they need to offset this loss somehow or they will loose money over time. Some of them will do this by creating great content to get revenue from upvotes alone. But for a large portion of SP holders this will not be profitable enough to offset the costs of inflation.
So SP holders will start selling their upvotes for money. Probably posts that contain some kind of advertising. There is nothing that can stop that development. Ads are coming when adoption slows down.
And if you think about it... it's not a problem as long as the ads contain valuable products for improving our lives.
The future looks bright with steemit!