I think you’re right to be concerned about centralization if relay specialization is the direction we go.
Ideally, the barrier to entry for new relays to pop up would be as close to zero as possible. That way, it will always be easy to bypass relays that somehow manage to get captured by centralizing forces. If running a successful relay requires building up brand recognition, then the barrier will be large, and disintermediation of captured relays will be more difficult.