You are right, some southern markets have not worked out. But some places (Carolina, Dallas, Tampa) have worked out all right. Each of those places have won a Stanley Cup. And Nashville has built a solid, consistent team.
I do think Hamilton (Ont.), Winnipeg and Quebec City should get a chance at the NHL. The fan interest and support would make up for the lack of population base. What's the point of having a large metro area if 95% of them don't care about hockey and won't give it a chance?