That's pretty crafty, probably several ways to mix-n-match like that and come out ahead.
Fact is, though, locked phones haven't been preventing people from switching. Every 12-20 months people are free to shop or switch carriers, getting a heavily subsidized (even free if you go mid/low tier) upgrade...but people just don't switch. There's maybe 10-15% of the market, most of whom probably really can't afford a smartphone, that are constantly chasing a few dollars in savings. 95% of the rest stick with what works and aren't even interested in trialing to save a few bucks.
I just don't see people "saving" money buying an unlocked phone retail, or taking their 20+ month old unlocked phone to a new carrier to save a few bucks. After 18 months, even mid-tier phones that you can get practically free are upgrades.
If there is any real benefit to unlocked phones, it will be maybe in bringing down manufacturing costs when they don't have to make 4 different versions for each carrier. That assumes the carriers aren't going to demand branded and exclusive phones.