To be honest, I still don't understand why they needed to shut the storefront. They still need to host the games so people can access the titles they've paid for, and I would imagine that makes up the bulk of the cost.
The only reason that makes sense to me is that they have plans to shut the entire thing down, including access to purchased games, x years down the line. It's probably much easier to get away with that if even the newest purchase being removed is however many years old.
Not that that stopped Nintendo, they took everything down for the original Wii, purchases and all.