Partly. But you've been able to do the equivalent of:

    echo "|magic-program" > .forward

For a long, long time. Granted, it's a little unclear what happens if you manage to overload your server doing that, but in general MTAs are pretty good at taking care of your mail, either queueing and delivering, or giving up somewhat gracefully.

The trick is to make sure that the "magic-program" is simple and robust -- and that isn't quite as hard as most people think, if you don't require it to parse arbitrary emails -- just accept and parse clearly valid ones, and reject everything else.

Now, if you require to handle enough incoming requests that forking a process per mail is a problem ... you probably need to fix your architecture.

The fragility of parsing emails is a big problem though. But I agree that for the equivalent of webhooks it is the perfect platform, and simple scripts work. Except at scale.