With idempotence, you shift the problem from "deliver X exactly once" to "make it seem like X was delivered exactly once". In most systems, exactly-once is really "effectively exactly once".
That's my point. You are simply converting the problem to a new form, not actually solving it.
Hey here's a solution to the halting problem – always assume yes, and then figure out the edge cases. How do you do that? Well that's on you, I did my job.
In a distributed system that needs exactly-once delivery, implementing perfect idempotence is equally impossible.
Converting a problem to a new form that you know how to better solve, or at least hope is more tractable, is a time honored mathematical and CS tradition
