Why Can't we replace the backpack?

Storage space is something we're very conscious of and we're definitely looking to keep providing storage options and upgrades in the future. We've added things like the key rings, profession and class specific bags, bank size increases, guild banks, and of course just throwing more and more slots onto bags. Those types of additional storage options being added are pretty much a given.

There is however the ever present ceiling of storage, not in your bags, but of the physical realms and databases. Bag slots takes up a certain amount of space in a database somewhere, the more possible slots a player can have, the more database space is needed. The only reason we've been able to offer any of the in-game storage upgrades has been through realm hardware upgrades and management. It's sort of a back and forth between what can be stored and what needs to be stored, how and when to upgrade the ability to store more data that needs to be, etc. It's really not as limiting as it may sound, but it's something that has to be taken into consideration throughout the game design process.

I think it was mentioned at BlizzCon that an upgrade to the original backpack is a definite possibility, if for nothing else than to help with its feelings of inadequacy.