I ran my previous startup (Twitpic) completely on ads and it wasn't an enjoyable experience from the business side. It also does not (usually) align with the user's best interest. Ad business models requires attention to feed it, which in turn requires social media companies to build features to get as many eye balls for as long as they can on their app.
I also don't think paid only is the answer either for most cases. Some are ok with seeing ads in exchange to not have to pay to use. I'm curious to see if there is a balance that can be struck with services offering both options.
Brave's new search engine[2] is another example saying they will offer a paid option in the future. I used their beta and it's solid. I'd be willing to pay for it to be free of Google and help sustain it.
[1] https://glue.im/glue/glue-a-social-network-that-respects-you...