Agree with the majority of your assessment. Should be Murray's job as 1a for the foreseeable future, bringing in Williams for a little extra juice a few times a game. 
However, I disagree that Murray is "done". No, he didn't show out well last night in limited action, although the 8 yard TD run was impressive. He's always had a good nose for the end zone. 
Let's remember that, due to his limited usage over the years, Murray should still have some tread left. In 7 full years in the NFL, he has 1385 total touches, which averages out to less than 200/year. He averaged 4.5 yards/rush his last 2 years with the Saints, with a 4.2 ypc average over his career. He also can catch the ball cleanly outta the backfield, with 185 catches with 7.0 yards per over his career. Yes, he's 31 years old, but the man looked very capable the last two years. Are we to believe that he hit the wall so quickly?
By comparison, Bell is only 29 years old, but has 8 full years in the NFL. He has 1950 total touches, or about 244/year, and a 4.1 yards/carry over his career.....and hasn't cleared 4.0 ypc since 2016.  I agree that he is likely washed. 
I think Murray can still produce in that offense. Heck, he had only been with the team a couple of days when mustered into service Monday night. Didn't expect much from him. Again, Williams has some definite flash, but he looked horribly inefficient in ball security and pass pro, and quite possibly cost them the game. That looms large. Murray's gonna get his chance, although I am more concerned about the loss of LG Tyre Phillips than I am about Murray's abilities.