People who speak a lot on the phone would have to pay a big premium for this, but if you don't talk much on the phone, PAYG data plans in the UK have really cheap, sufficient broadband. I reckon here in the UK people do unlock their phones a lot and this keep the market very competitive.
It's not just carriers that dislike competition. Makers also want to artificially "differentiate" their phones. (Translate: try their best to lock you up). This is all fair and part of the game. It's users who can make the market more competitive by favouring less crapified phones and doing it clearly, or doing their best to strip their phones of crap.