If you forward emails automatically then you'd lose this accreditation. I suppose the solution would be an accreditatiom domain that forwards to your uni address only, but that's extra work now.
Additionally, making people who want to cold email work a little to acquire the current email address is actually a good thing, especially if they want to talk about something years old. I’ve generally had a lot more pleasant and engaging correspondence with people who worked out my email (say from a side project I develop pseudonymously) than ones who directly lifted my email from my professional profiles. So, expiring emails in papers generally isn’t a real problem anyway, and it’s basically never a hurdle if your target is still in academic circles. It only becomes a problem in this specific context of automated authentication (based on something not intended for that purpose).
Obviously our university isn't gonna make a 10k€/month contract just because some prof wants their mail forwarded to gmail. Especially not if they are not working here anymore.
Most universities use a portal of some sort for easy access to personal information and preferences anyway, so it shouldn't be too difficult to limit access for alumni to only allow them to change a few personal details like name / address / phone number and the like, plus email forwarding settings. I think the extra cost is negligible compared to what universities already spend on alumni like newsletters, conferences, dinners, etc.