Again, yes, in the US. In some Asian countries, you can walk into any mall and there will be any number of stores selling new or used cellphones all of which are unlocked.

> People don't see it as the "grown up" option.

Again, this is limited to particular cultures. In some countries, prepaid is the norm, not the exception. People keep their numbers and switch phones as they move from one phone to the next, and buy only as much load as they need. (USD 0.20 good for 10 SMS messages)