UUID are the way, GUID for those in Microsoft's lands.

Not only will you never run out but you don't need a round trip to the keymaster, horizontal scaling, clustering and vertical sharding even are all way easier due to that. This removes a single point of failure entirely when the keymaster is no longer needed.

Using ints for keys for profiles/users and many other things were needed way back when processing/db/disk/memory and performance from that were a problem, no longer.

Do your part, join the UUID revolution. Also, if you were a piece of data, would you not want to be unique across all the databases, storage and services? You've heard of Roko's Basilisk right? Do not disappoint.