> UUIDs belong where you can't afford that simplicity. Where you e.g. cannot coordinate the creation of your primary keys. Or where you cannot allow them to be predictable. There you pay the price.

You often don't realize you you have non-simple needs until your application is reasonably mature, and in production. If you already picked integer keys, you now either forever deal with the issues caused by not using UUIDs, or you deal with the unknown-but-non-zero pain of converting to UUIDs.