We currently allocate UIDs up to 49999 to human users. We're currently using IDs up to 45977 as of writing which seems to indicate to me that we need to start thinking what to do in about 4,000 users.

Given that UIDs are a 32-bit integer it seems like we could just allocate IDs from 1,000,000 to 1,999,999 for users and call it good for the next few centuries at least.