So idempotency is easy if your service does not do anything useful?
(Though I do disagree with the original premise too. Putting on a 'stateless' boxing glove won't mean there's no difference between punching a guy once or twice)
There are still side effects in the system, of course.
But what your database looks like afterwards is the important part.
Can you recover lost data, replay transactions, undo, etc etc?
A database on it's own is enough for most business applications.
If you haven't seen this yet, you're just rent seeking.