Sure there are old songs they will always play and never forget how to play (Streets, WOWY, Pride, etc.), but then every time a new album comes out, they don't immediately play all the songs. It took a while for U2 to finally play One Tree Hill in the JT Tour. In Elevation, it almost seemed like an eternity before they played Kite and Wild Honey. In Zoo TV, they never played a full version of So Cruel, and maybe only once or never for Acrobat.
So obviously, U2 have little rehearsal time. I would rather have them practice more new songs that revive old songs that we've already heard in other albums or bootlegs. As for the new songs, at least we will get to hear them live. It's not a good thing if U2 will keep on adding to their list of songs never played live.
My proposal, practice and learn and play EVERY song off the upcoming album, then if you got spare time, that's when to try to relearn some older song. But of course we all know that will never happen, and not every song from the new album will make it to the stage.
Cheers,
J