You might think that albums from the 70s would have outsold albums from the 80s, which in turn would have outsold albums from the 90s, etc simply because the older albums have been around longer and therefore would have had the chance to sell more.
The truth is just the opposite, as shown in the following chart. (Click to enlarge.)
Each decade had more big selling albums than the one before - until a wall was hit in the 2000s. The wall was the Net, as discussed in the prior post.
But what of the trend before then? Either: (a) the whole market was expanding from decade to decade; or (b) the market was concentrating (i.e. the big sellers were getting a bigger share of the pie); or a combination of (a) and (b). We can't tell from these data alone.