I too am with a university (50k users) and we chose to keep the users because they come back. Student becomes residents, residents become doctors. It's funny, even people that are fired are hired back by other departments. Universities are ... different.
We like seeing the old users because they call back after being away and we can see their account was deleted. I know where your coming from though as the drop downs get kind of big. You could always add a filter to your raise user list on incidents to show only active accounts. Deleting users/incidents/etc loses history for trending. Not saying I do a lot of that but we just went back 4 years to track student usage of the help desk to see how new policy changes are affecting us.
So before you delete that data might want to reconsider IF you should delete it. If you do I have no idea on your SQL, way beyond my feeble SQL knowledge. I got lost way back at ...
SELECT
tu.tps_guid
tu is a table? I don't see a join making tu either. I am sure it is my lack of SQL knowledge so maybe someone else better answer.