Who determines "stuck in an unused address"? Say I have FirstBits address 1Crypto. I go to jail for 20 years but during those 20 years people keep sending funds to 1Crypto. When I get out I learn that oops 19.9 years ago someone got 1Crypto changed to a different address.
Now lets make it more complicated. Right now the way firstbit works is every client, website, user who uses firstbits SCANS the blockchain to find the first usage of an address matching the prefix. There is no communication required to a central authority or a list of unregistered addresses.
Say genuinely 1gig could be reassigned. How do you notify every user on the planet using a variety of websites, exchanges, clients that 1gig not points to a different address? What if some of those users are running clients which aren't updated? They keep sending funds to the wrong address?
deterministic as in from now until the end of time the "firstbit" of every single bitcoin address can be determined with nothing but an algorithm and the blockchain. If addresses could be reserved, or assigned, or reassigned that wouldn't be possible.