I think a bigger issue than open source timing is the community governance necessary to shape a tool that serves the needs of a lot of stakeholders. Swift has lived under the shadow of Apple's (often secret) product priorities. The extremely poor backwards compatibility is why I invested in Rust instead, and I think Chris has expressed some misgivings about how the function builder stuff got added unilaterally. It appears what they're trying to do with Mojo is build a more coherent starting design with a closed process but the experience of client work, and then open having skipped over some of the teething issues. We'll see if it works.