While it would work in the sense that compromising the other systems wouldn't compromise anything else. It does leave that central service as a very very very

While it would work in the sense that compromising the other systems wouldn't compromise anything else. It does leave that central service as a very very very big and attractive target. It introduces a single point of failure (security wise) that lets someone compromise everyone at everything all at once. It would also make it a good target for warrants and subpoena to be sent to gather information about people without their knowledge (less of an issue, but still a concern for privacy).