I think above all the player base should have a quest to receive a larger backpack. I understand new players start with a 16 slot, and that's a godly amount of space at level 10. After 8 years, I've seen classes come and go, and content in game come and go. I just think that with 8 years of playing, this might have been addressed. Just curios as to why simple quests like this we're never introduced.
We'd love to be able to upgrade it too, but it's not as simple as upgrading it. It was coded into the system early in the development of World of Warcraft and isn't as easy as writing up some new lines of code. In addition, we're not sure that simply adding a larger backpack is the solution people really are looking for. You'll get a few more slots, but you'll most likely fill those relatively quickly.
We've made some progress in regard to storage such as being able to learn pets and mounts etc. vs. having to carry them with you all of the time. We have Void Storage now as well as changed over some currencies (over time) from physical objects to something that gets tallied in your character pane; little quality of life increases such as those over time. The developers continue to do their best to keep storage in mind when designing new content.
But, if you're anything like me, you probably have a difficult time letting go of items that are either sentimental in nature or items that you "might need later... maybe". I'm with you, but in that case, I try to "spring clean" now and then just to go through what I have and figure out what I truly don't need and can get rid of.
As always though, we're open to constructive feedback and discussion. :)