Jack Knife, maybe this is really the last tour due their age or health. Not due the "reasons" you mention. Once again i tell you: analyze more carefully the things...
(A) You say "they will still finish with the most successful U. S. and world tours because of ticket prices, I guess." Only because of this factor?? Do you think that a total audience's number of 4.5 million people, despite the 500,000 "sold but lost" tickets in Europe due the cancelations, will be disappointing? With these ticket prices and after so many world tours in the last years? Obviously, if an act can attract so many people with this prices, with a cheaper for 25% -30% tickets (just an idea...) could easily attract millions again.
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Why do you think the ticket sales for Giant Stadium might be fast? Just a year ago they played in New York to most people than any other act. Were fast the sales for the first northamerican round of ABB tour? Of course not, everyone remember the same discussion ("poor sales", etc) a year ago. Did they played in packed venues in the most of the cases? Yes.
(C) ABB sold worldwide better than a lot of Stones albums in 70s and 80s. And if the US sales is reason to quit, then should quit the 80% of the rock bands. Look at the whole sales...