If you can predict user IDs this is extremely useful when you're trying to come up with an exploit that might create a privileged user, or perhaps you can create some object you have access to that is owned by users that will be created in the near future.
When I say "I don't have to think about it" I mean I don't have to think about the ways an attacker might be able to predict information about my user ids which they could use to gain access to accounts, because I know they cannot predict information about user ids.
You are dismissing the implications of using something that is less secure than UUIDs and you haven't convinced me I'm the one failing to think through the implications. I know there are performance problems, I know they might require some creative solutions. I am not worried about unpredictable performance issues, I am worried about unpredictable security problems.
