Or just log all cookies and other localstorage against the domain of the top-level window.location which would achieve most of what a VM would with much lower overhead.

The only problem is that this would break some things like certain SSO systems, so you would have to implement a white-list to allow shared state, and the UX for that would be abused to nag users to whitelist everything. Most people would just click “OK” by default like they do with everything else, and those of us with more sense would have a new reason to be irritated by incessant nagging.