I can see running your own small relay for private family-and-friends networks. That would certainly work. But for a large-scale network looking to compete with the likes of Twitter you need serious hardware and bandwidth, and that's expensive.
Discussion
Exactly. Personally I’m less worried about paid relays and more worried about who’s paying for the free ones (long term). The “if you’re not paying for the product, you are the product” adage.
What would you build if Twitter didn’t exist?