I'm probably in the minority here but Here's what I see shaping up...
Benning drafted Demko and has contacted a veteran goalie (Miller, Hiller) because he isn't happy with what we currently have. Demko by all accounts is up with the big club in 3-5 years(assuming he doesn't bust) which is right around the time the rest of our prospects should be hitting their stride so the timing with the next core works better then any of Lack, Markstrom, Eriksson.
Benning has also cleared tremendous cap space to re- shape the team. Like it or not I think his plan is to throw a 4-5 year deal at Miller make him the starter and trade one of Lack/Markstrom (whoever he can get the best value out of) before the season starts. I don't think at this point either goaltender is in the long range plans of Benning as anything more than a backup/trade piece as he continues to acquire players that he is familiar with. The last two years of the Miller deal would conclude in this scenario with Demko backing up one season while learning from an American veteran goalie, then the following season a 50/50 split before Miller leaves/retires, and Demko takes his spot as the starter.
Now let me make this clear I'm not saying this is what should happen... it's just my gut feeling from the moves I've watched him make so far. I'm not sold that Miller would be the right path to go down however this scenario seems like the most likely to happen if the team wants to still be competitive throughout this rebuild. Who knows maybe Melanson could work some magic on Miller and get some useful years out of him!