Well, the ability for top selling albums, in my opinion, isn't impacted. It is just that there are no big sellers anymore as there were back in previous decades, even with similar or smaller overall markets.
Thats just NOT true, look at the following:
Top 10 selling albums of 2000 in the United States
1. No Strings Attached / 'N Sync ~ 9,936,104
2. The Marshall Mathers LP / Eminem ~ 7,921,107
3. Oops!... I Did It Again / Britney Spears ~ 7,893,544
4. Human Clay / Creed ~ 6,587,834
5. Supernatural / Santana ~ 5,857,824
6. Beatles 1 / The Beatles ~ 5,068,300
7. Country Grammar / Nelly ~ 5,067,529
8. Black & Blue / Backstreet Boys ~ 4,289,865
9. 2001 / Dr Dre ~ 3,992,311
10. The Writing's on the Wall / Destiny's Child ~ 3,802,165
Top 10 selling albums of 2008 in the United States
1. Tha Carter III / Lil Wayne ~ 2,874,420
2. Viva La Vida or Death and All His Friends / Coldplay ~ 2,143,928
3. Fearless / Taylor Swift ~ 2,112,179
4. Rock N Roll Jesus / Kid Rock ~ 2,017,905
5. Black Ice / AC/DC ~ 1,915,172
6. Taylor Swift / Taylor Swift ~ 1,597,316
7. Death Magnetic / Metallica ~ 1,565,078
8. Paper Trail / T.I. ~ 1,522,103
9. Sleep Through The Static / Jack Johnson ~ 1,492,466
10. I Am... Sasha Fierce / Beyoncé ~ 1,458,853
As you can see, there has been over a 70% sales decline in the top selling albums from 2000 to 2008. You don't have to go all the way back to another decade to see how heavily top selling albums have been impacted in the past 8 years. The top of the market is more heavily impacted by
FILE SHARING, OTHER WAYS OF OBTAINING MUSIC FREE OVER THE INTERNET, CD BURNING, ETC..
Top selling albums from the early 1980s WOULD NOT be able to sell what they did back then in 2009!
The more widely available any product becomes, the easier it becomes to obtain that product for FREE!
This decade
at the start, had the largest number of multi-platinum albums ever seen in the history of the music industry! The spread of the internet, CD Burning, and other ways of obtaining music for free has in just 8 years brought the market for top selling albums back to where it was in the 1960s.
500,000+ selling albums per year in the 1960s in the USA
1960: 16
1961: 15
1962: 37
1963: 27
1964: 28
1965: 36
1966: 58
1967: 61
1968: 75
1969: 94
500,000+ selling albums in 2009
18 But, were only half way through the year, so lets take a look at how many have sold 250,000+. That number is
48! Thats actually only half of the 1969 total, and more in line with 1965 and 1966.
The key factor that your not understanding is the impact of the internet, File Sharing and CD burning on the MARKET TODAY. Those things did not exist back in 1983. The overwhelming majority of people had to actually
BUY their music in 1983 where as most people today obtain it for
FREE.
You can have unusual exceptions to the market(big selling albums), when the vast majority of people still have to BUY the product. Today, most people don't buy music because they can obtain it for FREE.
Even when looking at your figures of the overall market, notice that 2008 at 500 million is smaller than 1977 at 510 million and only 30% larger than the market in 1973.
Its expected that the 2009 total sales will be 15% to 20% below those of 2008. That means just 400 million to 425 million for 2009. 2010 will likely see just as steep a drop as well, which will put it below the 1973 figure for the overall market.