This was their answer a few months ago:
Quote Originally Posted by Grekumah View Post
Increasing both the amount of items that can be listed on the Auction House and the length of time items are listed on the Auction House will directly increase server load.

With that said, there are no plans to implement these requests, but if the dev. team decides to do so, it will only be one of the suggested increases and not both.

The development team would like to look into these requests very carefully because each one would increase the server load, which would also cause an increase in the time required for the Auction House to load.