I think you are over inflating U2s sales and even if those sales are valid which I have firm reason to believe they are not based on extensive hours of research at least 25 percent of those sales if not more has come from Best Of or Greatest Hits albums.
Well let see:
Best Of 1980-1990: 17 million
Best OF 1990-2000: 6 million
U2 18: 4 million
27 million albums of compilations sold which is 20% of the total since March 1987 of 135 million. If anything, the total sales number is a little less than what they have actually sold. The sales figures come from the record company and are often a little out of date. The majority of Boy, October, War, Under A Blood Red Sky, and Unforgettable Fire's album sales came AFTER the Joshua Tree was released.
The record company puts total album sales at over 145 million, but I would put it at 155 million based on the confirmed sales figures that I have seen.
Secondly The Joshua Tree came out 22 years ago and Achtung Baby came out 18 years ago, which in music equates to about 5 generations. Even ATYCLB is ancient in music terms for most of the music buying public.
How many people that were in the crowd at the GMA performance actually had heard more then possibly Beautiful Day and Vertigo? Not many considering they were not even born when U2s "Greatest Hits" were out.
Thats just an assumption on your part since I suspect you were not actually there conducting an interview of each person. Also, you don't actually have to be born before a song is released to have actually heard it, or purchased the album it comes from.
Plus, your forgetting the second biggest selling album of 2005, HTDAAB, or is 4 years ago to long to count as well?
Now we have 2009, and the biggest selling album worldwide so far this year is "No Line On The Horizon", and there is a decent chance that it will still be the top seller for the year come December.
U2 is 50 years old unfortunatly you cant be forever young and that alone will make some of the music buying public sway away from U2.
So why do they continue to have the top selling albums and tours of the year? Again, No Line On The Horizon will likely be the biggest selling album worldwide for 2009. By the way, U2 is Adam 49, Bono 48, Edge 47, Larry 47!
The last time Eno and Lanios were the primary producer on an album was Zooropa and that was an eternity ago. Unfortunatly most of the people buying CDs are used to the more mainstream "safe" sounding U2 and that wont help them on this album as far as sales go.
Correction: The last time Eno and Lanios were the primary producer on a U2 album was All That You Can't Leave Behind in 2000. By the way, Lanios did not work on Zooropa, it was only Eno.
Again, 1.7 million sold worldwide in the first two weeks and its already the biggest seller of 2009 by a half a million copies!
U2 need not to worry about sales anymore and simply base success on artistic merit of the record although I know they will have a hard time doing that. The hardcores will always be around.
Since the band formed in the late 70s, they have always tried to produce the best music possible and then to sell it to as many people as possible.