That sounds like a speculation on a tangential benefit instead of the major consideration in locking phones. The carriers can blacklist any phone's IMEI at any point (in addition to the usual collections attempts, credit reporting etc) which achieves the same effect but better if a phone is stolen from them.
It’s not speculation - this is literally why cell phone locking was invented. The imei blacklists were created much later specifically for theft, not to manage credit risk of customers getting subsidized phones.
You can literally just call your carrier to ask how to get your subsidized phone unlocked. There’s no need to speculate- it’s not a secret!
To be fair, that was an argument that was put forth by carriers years ago (at least in Canada). That said, I don't think it has popped up as much (or at all) in recent years.
