JS isn't today truly asynchronous in most usage today, but they are planning for that future (present if you count Service Workers and Node/Electron).
Try googling "fail fast" if you want to see more reasons to use this principle.
> Tearing down an entire application immediately due to an asynchronous exception has the possibility to leave the system in a deadlock or worse.
It should not. Because application can crash anyway - for example, if there is memory access violation or out-of-memory error. And synchronous applications crash on uncaught exception, I don't see why asynchronous applications should not.
Sure, fail fast is very useful in debuggers to help a developer find and fix problems. But this is why we have debuggers. Debuggers are a terrible user experience, we don't need to force every user to live in a debugger and decide themselves which errors matter.
Not every exception is "fatal" and "might look like it is working ok" is often good enough in the pragmatic real world where you can't control everything.
Also, just because you've never accidentally caused a machine to BSOD on a deadlock or catch fire from an improper halt in a "fast fail" of multi-threaded code, that just means you are one of the lucky ones. There are disastrous consequences to bad multi-threaded code during "fail fast" teardowns of threads not privy/cause to the error. There are enough nightmare scenarios to keep a multi-threaded programmer up at night.
I appreciate your concern that developers sometimes ignore errors they should fix instead or handle more appropriately. "Fail fast" in a debugger works to help that. "Fail fast" in day-to-day operation isn't the right solution, at the very least for making users possibly hate you because you'll never properly handle 100% of exceptions.
Also I remember reading somewhere that the earlier you find the error the cheaper it is to fix it.
> why did my app just disappear, did I do something wrong?
Usually operating system reports that there was an error in the application. If the application crashes often, the users will complain to developers and they will fix it.
> Debuggers are a terrible user experience, we don't need to force every user to live in a debugger
You should test your app properly before releasing it.
> There are disastrous consequences to bad multi-threaded code during "fail fast" teardowns of threads not privy/cause to the error.
I cannot agree with this. Usually the OS releases all locks, closes all files, frees memory when the process terminates. I don't see what can go wrong here and how you can get a deadlock because some process has terminated. Also if you use locking you should add some kind of timeout. For example, MySQL has lock timeouts and can even detect and fix deadlocks.
> "Fail fast" in day-to-day operation isn't the right solution, at the very least for making users possibly hate you because you'll never properly handle 100% of exceptions
The users will probably hate you more if the app will silently hang or show incorrect data.
> Catching unhandled errors is by design and it works great.
I don't think that it's great because by default I would prefer to terminate the application if there is an unhandled error or rejection. This is called "fail fast" principle. What promises developers suggest is to continue running the application even if it's broken.