The real issue here is security. Having all of those long since forgotten objects lying around the directory is a security nightmare. So many places for someone to exploit. Unfortunately it’s hard to talk to the higher ups without tangible numbers with regards to potential security risks.
The second thing to consider is performance. I don’t have any hard numbers on the decrease in performance with respect to Active Directory Size. But with most Microsoft products…the more you use them the slower they get. So I would have to say with THOUSANDS or inactive objects you would surely notice some performance gains and would give you some more time before you need to start upgrading the Domain Controllers and associated servers.
And I would suggest fixing the problem if you do decide to move forward with the housekeeping. Implement Windows and Active Directory auditing rules and track stale objects with reporting software. Enforce policies with Human Resources to find out who has been hired/fired/title change. Do simple audits monthly like printers and DNS Entries or stale accounts.
Good luck..hope this helped.