So you give another kingdoms access to your castle, but they are not allowed to touch things? Or they get details like your blueprints and who is stationed where?
Just kidding, I know there are ways to do this in a more complex and secure way, e.g. with self hosting services etc., but that is why I tried to not make it about all AV products.
The fact remains that this kind of layer can (and repeatedly does) introduce weak points that will be attacked. I am not saying there aren't ways to implement this well, with trade offs that are worth it. What I say is that: "Just add AV-product X" is likely a doomed approach.
Many clouds now support scans of snapshots, removing the need for direct access to the read/write internals of a workload.
This is where your analogy falls flat a bit.