I have a track record of being a trustworthy person. The risk here is losing the key, which can happen by mistake. It's not a sustainable long-term solution. I am aware of that fact, and so are many others. It's a short term solution to enable collaboration across protocols.
Having multiple bridges would not be as bad as you think. It publishes events with NIP-48 proxy tags: https://github.com/nostr-protocol/nips/blob/master/48.md And bridge servers ignore any events with this tag to prevent duplication and loops.
Clients would have to use proxy tags to deduplicate content if there were multiple bridges. And I am currently building a client specifically to do that (among other things): https://gitlab.com/soapbox-pub/ditto