You can certainly take the viewpoint that letting Murray walk in the offseason ultimately torpedoed any chance Dallas had of making a playoff run this year. Myself? I'm going to take the viewpoint that they were screwed the second their criminally underrated Pro Bowl QB went down and were instead forced to start the woeful combination of Brandon Weeden and Matt Cassel. Lance Dunbar was also playing extremely well before (surprise, surprise) he was lost for the season due to injury. I don't think Murray would have made a damn bit of difference to their overall fortunes this season and I certainly don't believe the Cowboys are kicking themselves that they didn't commit to paying him all that money. They'd rather have Dez, and rightfully so.
However, Miller won't be commanding Murray money, so I don't think it's a fair comparison, money wise, I also think that Miller is a top 10 RB, so I think we will actually be getting a bargain on Miller on the talent vs money ratio, and that's a good deal when available, but we'll see what Miller wants after the season.
I think that what you wrote, which I highlighted, is one reason the team has floundered. They cannot identify the talent they should keep, and the talent others have they should acquire. That being said... If you have a good running back, with this talent evaluation record on this team, you do NOT let him go unless he demands outrageous salary. You keep him, build the line up, and let'er rip. This falls under the euphemism... If it ain't broke, don't fix it. There are other things more important to fix than QB and RB on this team - LT, LG, RG, MLB, CB, CB, and one S for openers!! This does not even mention the question at HC. Arguing about not retaining a player who would do better with more work than worrying about players who should be reduced to little if any work (most of the OL) seems not only counter productive, but dumber than forty pounds of hammered horse ****. The Cowpolks let Murray go for economic reasons, to be sure. But we do not know what, if any, demands Miller has even intimated to anyone. But considering a good running back as a fungible good could only become real if EVERY team had a RB with a 5.1/carry average - which just ain'tagonna happen in this lifetime. So, if you got one, you keep him, until he gets ridiculous with his demands. A dismissive attitude is usually going to attract some attention - most of it negative.
Not saying it won't happen. But they haven't even approached Miller for an extension. we should have signed him this past offseason.
After watching the game today, why would any decent player want to remain on this team? Miller surely can find a playoff caliber team to play for who will pay him the money to be their starting RB.