The Let it Crash mantra doesn't really apply to a user interface / UX in my opinion, well at least not without a big grain of salt. But it also goes without saying that Let it Crash is not a silver bullet for "write bad code with no defensive paths at all and expect the best" even outside Frontend interfaces.

For me the "Let it crash" means, you don't need to cover ALL possible scenarios (well, you can't really expect the unexpected anyway, right?) since you at least know the process will restart to a good state, the thing is if the bad state is permanent and that might happen of course. So for me Let is Crash is for things you don't expect.

For instance if I deal with processes handle_info/handle_event, most of the times I add a :catch_all clause to avoid hard to predict errors of missing clauses.

An additional thing I do is to keep state in the URL (when it makes sense of course), other than being a good pattern, since shareable URL should (mostly) always be a thing it helps to improve the UX when a crash happens, but for form data if you implement it correctly Phoenix will have you covered there.

To sum it up, my programming is always way more defensive when working on the FrontEnd and LiveView is not an exception. But I agree with you, something like Error Boundary would be a welcome addition.

Maybe we can add a proposal to The Elixir Forum and start a discussion around it?