It might not be necessary or useful to have multiple aggregators right now. However, I do feel better knowing that if Bluesky the company goes under or changes to a degree where I'm not happy with their decisions, it's possible for someone to stand up a second aggregator.
For that matter, if someone's a free speech absolutist and if they care enough about it to spend the money, they could stand up an aggregator right now with more permissive standards.
Running an "AppView" (an atproto application aggregator/indexer/API server) is generally an order of magnitude more expensive and complicated. But still not beyond the reach of a user coop, non-profit, or small startup.
So these services should all be well within the capabilities of at least multiple companies operating in the atproto ecosystem as it scales.
And in many cases it should make good sense for these companies to do this since it will improve their performance by colocating their services and enable them to do things like schedule their own maintenance windows, etc.
It's not doing much to create the trust needed for this project.
The reference implementation: https://github.com/bluesky-social/atproto
Complete React Native Android/iOS/Web app: https://github.com/bluesky-social/social-app
Go libraries: https://github.com/bluesky-social/indigo
PDS distribution: https://github.com/bluesky-social/pds
All dual MIT / Apache 2.0 licensed.