Judging solely by mores law, it would probably be safe to have a max block size of 8 MB 2 years ago if 1 MB was safe 7 years ago.
Except that we fell off the curve for Moore's law years ago. Technology is struggling to grow at even 1/4 the rate Moore's law predicted now.
Looking at it strictly from a cost/unit, then yes you are correct. However in recent years, manufacturers of computer related products have invested less in being able to increase the number of units in the products they sell, and have invested more into making the products they sell more reliable, and otherwise increasing the utility of their products.
Take hard drives for example, while the rate at which the price per GB has declined has fallen, hard drives have failed at lower rates, and can access data more efficiently. This makes comparing the cost of a 1 TB HDD today to the cost of a 1 TB HDD yesterday not an apples-to-apples comparison. There is also the issue of the fact that from a retail user's point of view, there is little reason to need 2 TB worth of space on your HDD, especially considering that things like moves and tv shows can easily (and cheaply) be streamed as the user wishes to watch them -- this fact creates less pressure for manufacturers to create larger HDDs, which keeps somewhat of a mesh floor on the unit cost of HDD space. If a retail consumer has to choose between two hard drives, one with 500 GB worth of space, and one with 1 TB worth of space, he may choose the 500 GB HDD, even though the unit cost of drive space is higher because he has little/no use for the 2nd 500 GB worth of space.
Also, as HDDs become more reliable, consumers need to purchase less of them in order to keep their data on a HDD, along with necessary backups (or cheaper backup options can be used if as the chances of failure approach zero). Having to purchase HDDs less frequently and having to purchase less HDDs for backup purposes further skews the apples-to-apples comparison of unit costs.
If and when there are more real world retail demands for larger HDDs, manufacturers will invest more into creating larger HDDs, and I believe the unit cost will continue to fall at an accelerated rate.