The PDS design is a nice approach to making AT Proto distributed. But, the way they chose to operate relays means that the protocol requires massive investment to run.

The absolute minimum spend for a node right now, even with Bluesky still growing, is something like tens of thousands of dollars per year. If Bluesky does become the next Twitter, it will probably be $100k+ per year to run a relay. That means a company could run a relay, maybe a university could run one, but it's way out of the reach of anyone but the richest of individual users.