Zig is fast, promising, compatible with existing C libraries, and relatively barebones. The language itself may not be ready for production, but the binaries built in the language work just fine.

If Zig dies tomorrow, bun could probably continue using it as-is, perhaps after fixing the bugs they encounter. It's "the API and language spec isn't complete yet" unstable, not "we haven't implemented floating point operations yet" unstable. So far, only the allocalypse has caused major grief in terms of language changes, as far as I know.