I think that search engines focusing on decentralized networks or protocols would be highly popular.
As a new user of Mastodon and ActivityPub I was looking for popular people. I was figuring out how the medium is used at its best before personalizing who I follow. With the fully decentralized Scuttlebutt protocol the discovery of successful, public content is even harder.
Most pages I navigate to on the World Wide Web, I found in a search engine — even the ones I should know the address to.