That’s an interesting observation. Discovery is such an interesting aspect of these services (as with many content services, including music streaming). There certainly are people who want discovery as an element of their experience, and it would be a shame to shut them off from that. But at the same time, discovery often takes a significant time investment, and can be offputting if it’s too time-consuming. For that reason, I would think Twitter would want to keep discovery within such an interface to fairly narrow bounds, such as recommending new sources within those topics, or recommending new topics as a whole. That would make sense and stick with the simpler structure I’ve described here.