There's a very common problem with systems that use SSO, where the 3rd parties that accept SSO logins cache the login information, sometimes indefinitely. A user can leave the company but their login placeholder account stays in the 3rd party, and active login sessions are maintained basically indefinitely. So you can leave the company and lose your AD account, but still access the 3rd party. As Rachel says it's kind of a hard problem to solve (but not that hard).
In most cases wouldn't that session info be tied to physical hardware to which the employee no longer has access? Sure, tick all of your boxes, but I would think that losing the company laptop/phone/VPN would be a pretty significant barrier to maintaining access to other systems.
I refuse to BYOD, so I am not familiar with the nuances, but wouldn't the corporate controlling entity wipe/reset/deauthenticate the corporate partition of the device?
