T-Mobile, AT&T oppose unlocking rule, claim locked phones are good for users

Locking phones is not at all a requirement for carriers to offer subsidized deals. They could offer phones on installment plans conditional on an N-month contract. The buyer could switch carriers and keep the phone, but be on the hook to pay off the rest of the contract term. The only reason to use technological locks is to further trap a customer into a carrier relationship beyond the legal terms of their contract. It's yet another example of companies violating long-standing rights and norms and getting away with it because there's a computer involved.

What happens when the buyer says "no, I'm not paying" and sells the phone?

Instant money. Meanwhile the telecom company has to sell the debt at a massive discount to a collections agency or spend a ton of money collecting on it. That's assuming it can be collected on at all from someone that might just be running a scam.

With a locked phone, the phone just stops working and loses most of its value.

A rule forcing carriers to unlock phones after the term is up is fine. Forcing them to do it before is illogical. How many people are going to pay for two plans on one phone because they didn't like the first plan? I doubt it's more than those who will immediately abuse this rule and stop paying for the phone. I don't see the benefit to society here.

Being able to use a second carrier without cancelling a contract with the first is useful for international travel; the 'roaming' rate is often much higher than the normal rate of other countries' local telcos. Plus, mixing-and-matching plans can be advantageous to the user in certain situations even within one country. For example, data-only plans are typically cheaper than data+calls, but one might still want a very modest calls plan for infrequent use - sometimes, better value can be attained by combining two separate contracts. Of course, whether or not telcos ought to allow such use of their contract handsets is another question entirely!