Or.....
You can just ignore them for, say, the better part of 2 decades. Then when new staff comes along, such as myself, we can try to decide what to do.
That's literally the situation I found myself in.
If I had to start over, I would give them a notice about the expiration, allow 30 days to come current, then expire the file.
Also, for projects that have been started and are being expired, I would have Code Enforcement or one of the inspectors keep an eye on the project to see if work is being done without a permit.
It's tough when you let it go too long. We are looking for a legal solution that is fair and resonable, but also one that doesn't reward non compliance.
For instance, we have a number of permits that got up to the final and had a list of items. The people just decided not to correct those items or didn't think they needed something like a guard rail. I don't think it's right to final the permit, rather another solution should be found.
Good luck and let us know what you come up with.