I'm not sure where this notion comes from.
The tours before War were far less static in their closers/encores, but certainly Out of Control and 11 O'Clock Tick Tock>The Ocean saw heavy duty in the "final song" slot on both the Boy and October tours.
40 was the standard closer for the War, Unforgettable Fire, Joshua Tree and Lovetown tours. There were exceptions, but 40 was the rule.
Love Is Blindness or Love Is Blindness>Can't Help Falling In Love closed all of the Zoo TV and Zooropa shows. Between the final Zoo TV Outside Broadcast show on Nov. 25, 1992 and the first Zooropa show on May 9, 1993, more than five months passed, and the band added an album's worth of new material. So while the general show and setlist was very similar, I don't think you can call the 1992 and 1993 tours the same. And they certainly used the same closers for both.
One was the sole closer (with add-ons) for PopMart. And really, the lack of continuation of a closer from PopMart to Elevation is the ONLY time in U2's history that it hasn't used the same closer in more than one tour.
I would argue that Walk On is entirely possible as a closer (it was a very good one, too).