...I don't think VLV would have done much better than NLOTH had it been released in March 2009. The economy world wide is down and there was no Christmas boost for NLOTH like there was for VLV. The #1 album of 2009 is likely to sell a lilttle over 4 million copies compared to double that last year.
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In any event, sales of CD's will likely continue to plummet in coming years and become increasingly irrelevant. No one I know who is under 30 even buys CD's anymore.. they just d-load the songs or share them with their friends. There's no reason to think they are going to start buying CD's, especially at the extremely over-inflated price of $11 to $15 a CD...
I agree with you that the crisis and shift towards digital music are explanations for the decline in sales. But I guess you mean the illegal downloaded music. (see also the comment of Maoilbheannacht).
However this is not the full explanation for the 'disappointing' numbers for NLOTH. In Netherlands (I don't know about the ROW), physical CD sales dropped 8% the first 6 months of 2009 compared to 2008. 8% does not explain a 50% difference in album sales between 2008 and 2009's best selling album.
Coldplay sold ~730,000 albums of Viva... in the first week (in June) in the US, which is similar to what they sold the first week for X&Y, in 2005 (also June). Coldplay sold ~300,000 copies in the first 3 days in the UK (~500,000 in the first 10 days, more than X&Y, 2005).
There are 4 years in between and they sell similar amounts of albums.
The big difference is that Coldplay had big hits played all over the radio. That's probably more important than timing the album for Christmas release. (Coldplay released it in June)
KOL was released closer to the Christmas season, which probably helped sales. However, that album stayed in the charts for a long time everywhere. For example, it debuted in the US at number 5, while after staying around 50 weeks in the charts it was number 4 again!!
In the UK their album reached three seperate times the number one position and stayed in the charts for over a week (on high positions!)
Also, Only By The Night (released 5 months before NLOTH) is STILL in the Billboard 200, at number 23!! (59 weeks!)
Viva La Vida (released 8 months before NLOTH) is still in the billboard 200 (174, 73 weeks)
The big difference is big hit singles !