If search engines were 100% relevant…

There would be no search results. You would only need the I’m Feeling Lucky Button.

Every page on the internet would be a stop page. The searcher would use the SE to find exactly what they were looking for on the page of your site the SE sent them to and would no longer need to browse any other pages on your site.

Affilliate programs would not exist because if searchers needed to buy something the SE would send them directly to the people who sell them.

Internet traffic would drop dramatically because hunting for information wouldn’t be necessary.
No new ecommerce sites would emerge to compete because the SEs would automatically send all the people on the Internet to the most reputable online stores.
There would be no need for sites that review other sites

Directories like DMOZ would no longer exist

No sites would link to other sites because the visitor wouldn’t be at your site if it wasn’t exactly what they wanted.

Forums and chat wouldn’t be needed because finding the information would become easier than asking for it.
The web would be dead.