well as Bungie said, there is a limit of what they are able to do in regards to programming such things without doing something else to compensate, like the original vault space increase (xbox 360 and ps3 had to give up gear comparison when using the vault). So it really is going to be a little difficult for another vault space increase if something has to be changed again as a result.
It HAS to happen though. Can't change that little kernel of truth. And I'm sure you're aware of upcoming dlc? What then?
There was simply too much weaponry and armor introduced in this game to keep on all Guardians and vault space...
They can toss out all kinds of statistics telling us "look, look, you guys actually DO have vault space left over according to our numbers, so guess you don't need it very much...Also, is hard for us to conjure up more space", but the fact is that people are running out of space. Namely collectors and hardcore players.