I agree. Lack wasn't playing well near the end of last season (although he was severely overworked) and Markstrom was coming off an average season in the AHL and his few games in the NHL. Goaltending was a very big risk for us going forward.
Bringing Miller in stabilized our goaltending situation and allow us to send Markstrom down to get more playing time in the AHL. Look at Markstrom, he is playing very well in the AHL regular season and playoffs and will become at least a decent trading chip for us this upcoming draft.
Although I agree signing Miller to such a long and high cap hit contract might not be the most ideal plan, people should give credit to Benning who managed to slip Markstrom (throught waivers) into the AHL and allowed him to develop back into a good goalie prospect at the minimum. People also need to remember that as average as Miller's number looked, there are a few games (esp. early parts of the season) where Miller absolutely bailed us out.
At the minimum, signing Miller gave us an extra trading chip to fill some of the other bigger holes in the club. We need value other than in the form of draft picks in order to be able to get value from other teams. Sure giving up Lack (or Markstrom) might sting, but at least we can get some decent value for positions where we have a little extra depth (which is not something this organization can say about its defense and forward.)