You're implying that reckless rewrites within the JS ecosystem are a novel event, or more specifically that surprise language changes over a short period of time are. And yet... I can think of at least six times in which exactly this has happened and little fuss was made because the polarizing element of "AI" was not involved. Not just JS to Typescript, but to Dart, Go, C, Rust, Zig, Nim etc.

From any reasonable perspective, this is business as usual in the house of cards we all operate in. Perhaps the sensationalization would be justified if the lang migration wasn't one of less correct -> enforced correctness by default?

To your point in general about maintainers holding a mental model of the runtime: I would challenge that to say that it is very likely that there is no developer who holds a complete mental model of an entire runtime at any given point. As with anything of this scale you understand individual parts in their entirety and have general assertions about the rest until specifically revisited, even if you are the sole developer. In this case specifically, Bun has been largely AI driven for quite a while anyway so it is even more unlikely that the developers ever had a complete picture in the first place. If you trusted them before, then nothing has changed.

It's not lost on me that code logic can be subtly incorrect even as tests are passing either, but there isn't exactly a lot of grey area in this particular context. Does your code compile or not? If it builds as expected, then your own unit tests will highlight the difference.