All Verizon phones are automatically unlocked after 60 days and Verizon has still been financing phones.
> When locked, the phone is the collateral
Not really. If someone cancels service without paying off the phone, the carrier doesn't reclaim the phone. It simply prevents the phone from being used with a different carrier. The person could sell the phone to another customer on the same network. Houses are collateral because it's hard to hide a home from creditors and you can't sell the home without discharging the lien on the home. There's no lien on your financed phone.
This change probably wouldn't change much for phone financing because phone companies are already running credit checks when handing out devices on payment plans, require higher-risk people to make down-payments on the phones, and once a person has done it once to you, it's easy to never offer it to them again. Once you've burned Verizon by canceling service and not paying off your phone, you've burned that bridge. Plus, Verizon would likely report it to the credit agencies where you'd have burned the bridge with the other carriers too.
This is a rule that would help prevent a lot of e-waste and make it easier for folks to switch carriers. There is the chance that someone will finance a phone and leave without paying it off, but there's always been a risk that someone would run up a phone bill and not pay it. Someone could go abroad and run up a large roaming bill and not pay it. Back when data plans were limited, someone could run up a bill into the thousands and just cancel service without paying.
Locked devices do serve a function for carriers. They make switching harder and they protect companies roaming revenues. If I have a locked phone, I have to pay Verizon $10/day to use my phone in Europe. If I have an unlocked phone, I can grab a European SIM for $30 and have cheap service for the month.
Verizon has been financing phones and offering similar discounts that T-Mobile and AT&T have been offering even though their devices will automatically unlock after 60 days. A locked phone is worth marginally less than a carrier-locked phone, but a locked phone can still be sold to other people, even if it isn't paid off. A locked phone can still be used even if the original purchaser has defaulted on the debt. These aren't bricked phones, just carrier locked devices.
If the issue were that they needed a form of collateral, carriers would want the ability to brick the devices rather than merely reduce the resale value of the device by 15%. Yea, if you finance an AT&T iPhone, default on the debt, and sell it to Gazelle, you'll get 86% of the price for your locked phone as you would for an unlocked one. If one could flip unlocked financed phones, it would be just as easy to flip locked financed phones - you'd just make 15% less per device.
So what is the lock preventing? It's not preventing someone who has found a way to defraud carrier financing. They can still sell the phones (which are legally their phones which the carrier has no lien on). They merely get a marginally lower resale price. No, the locks aren't necessary for equipment financing. No, the phones aren't collateral.