Drones could not be seen or detected, hence used as assassination devices. Iran is successfully killing drones, they are no longer immune to detection.
The US wasn't using drones to assassinate people in Iran. And so what if Iran can do it? It's not the same as someone elsewhere achieving the same feat, particularly without creating a military target in the process. Keep in mind that the US strategy is to always have drones in the air. So it's not that useful to be able to detect drones, because you will always be able to detect drones. Merely detecting drones tells you nothing about whether the controllers of those drones know enough to commit an effective assassination strike.
Study up on DOD and Military expenses, money has never been an object, ever in the history of the military.
The Second World War and the Cold War are obvious counterexamples. The US won both wars in large part because its opponents could not afford to match the US's industrial/economic strength. I don't see that working at all with China in the next few decades. And even ignoring the competition from that emerging superpower, we still have that the US military and its ongoing activities are a huge drain on the US.
To the last part, I think we are close to agreeing except for where you claim autonomous systems would still require humans.
I think a completely autonomous system will eventually be feasible. But then how would you know that it is working as intended? That implies the involvement of independent sensory systems, which would eventually have humans in the loop.
Further, it runs completely contrary to how modern military systems and strategies work. A key aspect of US military development is the combination of improved intelligence of the enemy with more accurate and precise delivery of military force. Manufactured ignorance which impairs decision makers' knowledge of ongoing military activities runs completely counter to that approach. I don't buy that anyone would allow the entire US military to run autonomously and unsupervised (especially given the many constraints that have been put on the US military over the centuries) just so they could have marginally better plausible deniability when it comes to killing innocents.