You cache the compressed packages, while there is (say) 20% of disk space free. If you do a reinstall you used the cached packages (after a checksum which it should be doing anyway !) rather than redownloading them. If you don’t want the cached packages you just delete them (or you could add an option to turn it off).
So if you have to do a reinstall it should be much quicker.
It would not always work ; Asobo occasionally consolidate fixes and that means the download suddenly drops from 140Gb to 90Gb, and then the downloads are out of date.
Ideally a validation option would be added but that is way more complex because of the interrelating parts of the various fixes. Caching is relatively trivial. Most of it is there already ; it downloads the .fspackage files to a directory somewhere, it presumably checks they’ve downloaded okay. So all you really have to do is check if it’s already downloaded and if so use it if it passes the check, or delete it and download it if it doesn’t, and make the directory visible.
The other potential issue is piracy, people could, theoretically, copy the .fspackages and reinstall them, but if that works people will pirate it anyway.