you are making the assumption that someone only has a single computer connected to this link.
The averge geek home has between 1 to 3 computers devices, add in the support for ondemand TV over that internet link, and well, you can start to eat up a good amount of bandwidth.
Now imagine someone with a big or extended family.
I am not saying that any family would be able to saturate a 40Gbps link, but this link would probably not be the norm for any family for a while.