The author counts the storage on a yearly basis (servers to store a year of data). If you allow an expiration date for the records (let's say 4 years), after that period you can spend less on hardware, as you can free space from the old records. Then you only need to spend money on the traffic difference (as the traffic would increase in 4 years).
As the storage boxes in the article also have a nice CPU, the collected data can be indexed and then compressed, saving a lot of space.
Given that the Internet grows exponentially year by year, while the cost to store a bit of information drops in a similar fashion, I doubt there is any money to be saved by deleting old data. The save in system complexity is likely to handily outweigh the additional cost of storage, not to mention it isn't worth one iota of frustration and bad reviews if data an analyst wants is not available.
As the storage boxes in the article also have a nice CPU, the collected data can be indexed and then compressed, saving a lot of space.