You've bought into nonsense...
Here's what the "succession plan" should be. You keep the best goalie you have available - the best combination of quality, cost, and longer term availability. When another goalie proves to be a better combination of quality, cost, and longer term availability... he now becomes the best option, until a better option eventually unseats him.
To make decisions now, based on the idea that Demko will be on the team at some point in the future, is nonsense. Investing that much money into Miller, at the expense of having the difference in Miller and Lack's salary that could have been used to address the defense, is nonsense. Continuing with Miller next season (to become some future mentor to Demko... who may or may not actually become a NHL player), when Markstrom has now shown himself to be at least comparable to Miller (which is a rather average bar), is nonsense.
You are supporting the actions of a jaded and delusional decision maker. No decisions should be made now, based on prospects who may or may not become NHLers one day... What you especially don't do, is give a time frame for when the prospect will be in the NHL, and make decisions around that in the present tense. If Demko is ready to be the best option one day, we will all know... just like we will all know if he's not. To me, that there are some people arguing to keep Miller (and not using his $ elsewhere) because it's a variable in a logical succession plan is a loss of plot from people who have bought into nonsense and fairytales about the "Prince that was Promised". If Demko is the Prince that was Promised, it will become clear, and Demko will be worth making NHL roster decisions around, at that time.