I'm still using my student email from 2004 to get discounts
Why do you feel you are entitled to keep using that email to get discounts?
I'll pay less if I don't change it
I understand your desire to want to pay less, but it's wrong to do it deceitfully. You should stop using that email.
It's just the way the system works and I'm a beneficiary so why should I change it?
If you are no longer a student of that school then it's a bit wrong to pretend you are by using your student email.