@NigelFrobisher There actually are a bunch of competing fediverse platforms that use entirely different codebases. Some are actually older than Mastodon.
For example, I am posting from Hubzilla, not Mastodon. Hubzilla has an entirely different codebase and an entirely different feature set.
In the short term, I think that Bridgy Fed's multi-protocol bridge has a better chance of decentralizing Bluesky than Bluesky.