This rules out entire classes of app and would make using a computer a miserable experience.

For example let's say you want to make an app that every day writes a backup to a particular location e.g. 1Password can do a daily backup of your encrypted passwords to a backup location.

Or, let's say you want to make a GUI around a command line program that stores its config as a dotfile.

Without a way to save access to file system locations persistently, apps would be forced to constantly shove open panels in your face all the time.

Expiration depends on how the app has implemented the request for access. Granting access creates a security-scoped bookmark. The app can store it and use it the next time access is required which will bypass the prompt and the bookmark will remain valid in perpetuity (or until tcc reset), or the app can not store it and request permission every launch.

IIRC the bookmark is a base64 encoded plist containing bunch of data about the file/folder. A quick search got me this: https://www.mothersruin.com/software/Archaeology/reverse/boo...

“Should” meaning “I believe it currently does expire after a short time”?

Or “should” meaning “Apple should change this to expire after a short time”?

No, it shouldn’t. There are real reasons to give persistent access to a particular directory. Maybe you want your note taking app to put all notes in a directory for iCloud/dropbox/google drive/some other sync service.