If my predictions are correct, advertisers will only be happy in the short term; in the long term they'll leave too.

User engagement depends a fair bit on the quality and diversity of the content in a platform. And there's still enough content there to make your typical user happy; however that "old" content will become stale, repetitive, and boring. This wouldn't be a problem for Reddit, if it didn't manage to piss off the portion of its userbase that was the most prone to bring new content into the platform.

As a net result, user engagement will slowly go down. And since user engagement is directly related to the willingness of an advertiser to buy ad space in a platform, the advertisers themselves will be less willing to pay for ad space in Reddit. They'll leave unless Reddit makes the ad spaces in the platform dirt cheap, but by doing so it'll get less ad revenue. And if it tries to compensate this loss of ad revenue by creating more ad spaces, then it's further encouraging users to leave the platform, worsening the problem in the long run.