I'm sure this is happening but I also wonder if ironically those systems are hardened by adversaries. I could imagine nation state actors, once they're in, hardening the systems themselves so they can keep their presence and exclude others.
Using that access is the kind of thing that is only done when there's a need and in the case of really critical stuff that can be years in the future - or never.
One could even imagine hyper-critical exploits being shared back to the host. For instance if Russian agents got access to critical nuclear systems (or vice versa), I could imagine leadership warning the other side and getting out of the system entirely in order to maintain the status quo at the expense of pretenders.
Using that access is the kind of thing that is only done when there's a need and in the case of really critical stuff that can be years in the future - or never.
One could even imagine hyper-critical exploits being shared back to the host. For instance if Russian agents got access to critical nuclear systems (or vice versa), I could imagine leadership warning the other side and getting out of the system entirely in order to maintain the status quo at the expense of pretenders.