I feel like it’s really far from being open. Besides the training data not being open, the more popular ones like llama and stable diffusion have these weird source available licenses with anti-competitive clauses, user count limits, or arbitrary morality clauses.
Some other fediverse software like hubzilla and sharkey let you migrate posts, so I wouldn’t say it’ll never happen. I don’t think anyone is working on it though, so probably not anytime soon.