Basically the carriers are making the standard libertarian argument, which makes sense. If you block locking, you already know what happens: we already know cell phone prices unlocked. The cell carriers are in essence capital providers and they know how to collect money from their customers.
It really doesn't. Libertarian arguments only make sense if you don't think about it too much, or are ignorant about the context and details, or you have a vested interest.
You can compare phone prices with countries where there is healthy competition and there is no or very limited blocking (France is a good example - you can buy phones outright, or get them on a payment plan that locks you on a more expensive monthly payment compared to the classic 20€ everything included including 20-150GB internet depending on the provider plan; after the initial period is over, you can do whatever you want). If you bother to look into the topic a little bit more than surface level, libertarian arguments usually fall apart easily.