They had to. Who on the open market is better? If Tom Brady was younger then him obviously but at his age you just don't know what your going to get and there were cracks showing last season. Rivers showed serious age decline. Dalton is a wash with Tannehill. Cam Newton is a huge risk because you don't know if he's going to be healthy. They draft too far down to get one of the top rookies and besides which they are in superbowl now mode.
And as a long suffering Miami fan I can tell you that Tannehill never really got a good opportunity in Miami. The Joe Philbin era was a disaster as he was a bad coach. Then Adam Gase is the most overrated play caller in history whose reputation was built by Peyton Manning calling audibles at the line. You saw classic Adam Gase last season in New York when he misused Leveon Bell all season.