Here's where I think we disagree.
1) Leonard and Aldridge are multidimensional scorers. Mills is not. Leonard and Aldridge can score in a mul ude of ways -- Jumpers, ISOs, posting up, getting to the line, ect. Mills can just shoot. That's it. He can't create shots for himself with the ball without a screen AND lazy PnR defense. He doesn't have other options besides using screens off the ball or spotting up. Mills is not a multidimensional scorer -- he's an Eddie House, a Gary Neal. He can score yes, but he's handcuffed to only shooting just like Eddie House and Gary Neal were.
Remember when Gary Neal avg. 10 points a game, 3 years in a row, shooting the ball? Remember the guy who waived the towel a lot that was buried behind him in the rotation? Did the Spurs recover when they decided to not pay Neal? If Neal had Mills' character and at ude, Spurstalk would have been so against letting him go ( much like people are doing now about Mills). How would they ever replace a shooter? Neal was the exact same player. Great shooter with the screen, great spot up shooter, terrible defender, not a good creator. However Neal had a more selfish or quick trigger from time to time, but same skillset nonetheless. And the Spurs were just fine letting him go because they had a player making hardly anything behind him ready for a promotion ( Mills then -- now Murray).
2) I don't think the gap between Manu/Patty and their presumed future successors is significant, if at all. I think Simmons and Murray are close to Manu/Patty overall offensively, all while offering much more on the defensive end. You seem to think otherwise as you're implying there's a significant gap and Spurs would be a 5th-8th seed because Mills/Manu are better shooters ( all while glossing over all the other important parts of the game). This isn't 2014 anymore for Manu and Murray is much better and more ready than most thought he would be early on. He has better defense, which is half the overall impact, better play-making --which is very valuable offensively and his shot has improved to be at least respectable). I don't see the gap you're seeing.
3) I don't think David Lee is going anywhere, he's at an age where the minimum is going to be the reality for him. The minimum will be more than it is now, but I think he'll be stuck with it due to the market for 34-36 yr old PF vets off the bench. And I honestly think David is really enjoying himself in San Antonio, you can really see it with his body language on and off the court. And say I'm dead wrong and a team comes in and offers David a 3 yr/20 million dollar deal, Spurs have Bertans ready for an expanded role and they have Anderson too - no harm no foul there.
4) I don't think its a Murray OR Mills dilemma. I just don't think Mills is worth a 4/44-50 million dollar deal. Not close. Not with Murray ready for an expanded role for 1/12th- 1/14th of Mills' presumed salary. I'll wish Mills the best, great guy, great teammate, but its basketball economics. I don't agree with that decision to pay Mills, just like I didn't agree to pay George Hill -- when me and objective discussed (weeks prior to draft) and wanted the Spurs to cash in their chips for Hill by trading him at the draft to Indiana for their 15th before his last season under rookie contract.