This seems to me like premature optimization… but an easy way to overcome this could eventually be a relay whitelist/blacklist… so if you end up finding yourself too centralized, you can just add parameters to the algo so it ends up with a better list. Most people won’t care about this, but if enough power users do, it may end up creating new popular relays.

even if everyone in the network is following the same 10 relays, how is taking down one an easy attack vector?

i don't think everyone should have same/similar relays. i just think community conversations should be grouped on common relays, not randomized over a slew of relays