What would be the consequence to them if they did screw it up? Screwing up the maintainability of a project, especially a big one, doesn't necessarily have immediate consequences. The fallout could be delayed by a year or more. Also, they have effectively limitless tokens to burn on keeping everything looking OK, and a vested interest in doing so.
I'm not trying to spin up some kind of conspiracy theory here, but I'm not sure to what extent Anthropic does have any vested interest in this project (in fiscal terms at least) because the reputational fallout could be significantly delayed and might just not be big enough to matter.
Claude Code is the main reason their revenue (ARR) grew from $9bn to ~$47bn in the first half of this year.
That's a very big reason not to screw this up.
It's far less obvious how choosing Bun results in more revenue for Anthropic. Unlike Claude Code, Bun doesn't require you to pay for tokens to use it
Bun powers Claude Code. If the rewrite introduced catastrophic bugs, it would cause catastrophic bugs in Claude Code and therefore without exaggeration could cause $billions of lost revenue.
There clearly aren't catastrophic bugs. The question is whether the artifact is maintainable over time, since clearly there's a massive amount of net new code that was not thoroughly reviewed. Give it a year or so and we'll know. I claim either way, it doesn't really matter to Anthropic--if it's a disaster they might be able to keep it limping along well enough by just throwing tons of tokens at it. The value in this is not the lasting software artifact, it's the marketing of the rewrite. If over time it turns out to be a great success, all the better!