I can go and dig up bootlegs where they played only a five or six minute version of 40. Usually the band played it for a little longer than that and then the audience took over.
I should not have stated that U2 never played 15 minute versions of any song. Once or twice in the nearly ten years that they closed with 40 they played it longer. And yes there were occasions when Bono got delayed in the audience during Bad so the band kept playing and playing until he got back to the stage to continue. So I should have said that U2 never regularly played 15 minute versions of any song.
But, Axver, in your post you state that U2 should go back to playing 15 minute versions of 40 as though that were something they did all the time. They did not. I know that you're young and never saw U2 in the eighties and are basing your statements on bootlegs of their concerts but, as someone who saw them perform back then, I think that you're placing the U2 of the eighties on too high of a pedestal. They were great live back then. BUT they are great live now too.