Fact is a lot of people who went along in 2004 or 2007 will have "ticked the box" and seen a legendary act perform either after a long absence or for the first time.
There's not many artists that I'd go to see 4 times in a decade, so it's not surprising that attendances are dropping back to the hardcore fans, not just in Europe but North America too.
A good example from this tour would be the visit to Halifax, Nova Scotia, where they haven't played since Hold Your Fire. The gig sold out in 10 minutes and they had to add another night. I don't think that's happened in many other places where, if not contempt, familiarity certainly breeds apathy...