I think the real issue is that US customers (at least) have been conditioned to think of a smart phone as something that costs between $0 and $300 due to the common use of contracts and subsidies. Even if a carrier offers a non-contract plan, they are often not that much better as far as price (with some exceptions like TMobile and some MVNOs), so they balk at paying $650-850 for a handset.

That was one of the things I liked most about the Nexus 5 when it came out. Even unlocked you were getting a current gen phone, unlocked, worked with three major carriers and many MVNOs, and only cost $350. At that point, it's not as tough to make the jump and ditch the contract. Without the contract, you can just leave if a competitor offers a better deal or if your carrier screws you over.

Even with this new SIM, I imagine you're still either gonna pay full price or you're gonna get it subsidized with a contract and then have the option of flipping over to some secondary plan when needed. Might be good for smaller carriers with good rates but less coverage. Phone works great at home but drops off when you go to another town? Just flip to a prepaid SIM from a cheap prepaid carrier with service in the area.