I keep seeing the argument that there should be niche or specialized relays for various topics, such as politics, bitcoin, music, etc.

I think this idea goes against the whole point of Nostr. Relays aren't meant to matter or have any influence. Centralizing interests to specific relays will give those relays the power to control a potentially large and influential town hall. The largest politics relay, for example, could start blocking people with certain world views.

Relays are the fabric of Nostr, they need to remain interchangeable and individually irrelevant. The worst thing we can do is find ways to give power and influence to relays.