Creating a user account management policy to delete old accounts

Actually, when it comes to a user account management policy, I think you should have both a default deactivation policy for inactivity, along with a verification process involving local supervisors to ensure deactivation is necessary. If a user hasn't accessed an account after a specific period of time, the account should be orphaned -- the user no longer needs access but the account is still active -- and it's important to delete old accounts.

However, with that said, workers take maternity leave, projects get delayed due to budget constraints, employees...

come down with unexpected illnesses or need to care for loved ones, etc. No system can recognize these cases; only supervisors, and possibly HR. So, when it comes to people, make sure to consider the human issues, which are the domain of supervisors: They'll know whether an employee will return tomorrow, in a few days, or never.

In the worst-case scenario, such as you disable an account just in time to find out that the worker is returning the next day, and you need to re-enable it, experience says that re-enabling doesn't simply involve making a call, but rather following a process that could take hours or days. I'd suggest following your CIO's advice.