I can only go off my experience.
Stone Roses were better live at Heaton Park than they were at the Apollo in 95
New Order are far better live now than when I seen them in the 90's. That's even with no Hooky. Depeche Mode are a far better live now than when I seen them at their peak in the 90's. That's when they had Alan Wilder. The list goes on and on. It's the same for James, The Charlatans and so on.
You're looking back through rose tinted glasses, back to a time when you were young and you have cherised those moments.
But, if musicians play for multiple hours every single day, for 35 years, they get a lot better. They perfect their stage routine and put on a better show. They have better gear. It might not have the edge and vitality they gad when playing in the Hac, 40 years ago. But, they are better musicians. There is absolutely no reason why Oasis can't put on better performances now than they did at Maine Rd or Knebworth.
As for your Morrissey comment. The Smiths don't play together. Morrissey is not going to be better than the Smiths. Johnny Marr is a once in a lifetime guitarist, a generational talent. I guarantee you now that Johhny Marr is a far better musician now that he was 40 years ago.