What are the gains from competition among insurers?

Ezra Klein reports:

For people, like, well, me, who think that the health insurance
exchanges have a real shot at lowering health-care costs throughout the
system, the graph
above is difficult. For conservatives who believe that the key to
constraining health-care costs is to encourage competition between
insurers and give individuals the opportunity to choose, the graph
above is difficult. Because what the graph above shows is that neither
of those strategies has worked terribly well, at least as of yet.

The bottom line is that premiums seem to go up no matter what the institutional structure or its degree of competition.