No. They explained the problem in-depth during the engineering panel at the Blizzcon 2015 ampitheater discussion.
https://youtu.be/mx_C7LkB1Q8?t=750
The way player inventory was built by the original team was very poorly designed. Essentially, your currently equipped armor and weapons, bag slots, and bank slots are all part of a single linear array. The array starts with twenty slots that are assigned to your currently equipped armor. It then has a number of bag slots, which would be fine, but at a certain position in the array your bag slots end and your bank slots begin.
The fundamental problem with that is that the programmers didn't want you to be able to access your bank slots from anywhere in the world, and their (shortsighted) solution was to hardcode that position in the array into a number of different statements all over the code to prevent you from accessing it freely. Since it's a hardcoded value, the only way to change it is to manually go through the code and find and rewrite every piece of code that uses it. There are even places in the code where programmers used it in functions involving math logic, because it was treated as a constant value.
That's not to say it can't be fixed. It would just be a fairly difficult and QA-intensive project that would take resources (time, money, employees) that could be better used elsewhere. After all, there are other, easier solutions to the problem, such as reclaiming obsolete slots (ammo, relic, keyring), implementing alternative storage solutions (void storage, collection manager, currencies) and reducing clutter so that players won't have to use as many of the slots (quest items, dynamic stats, larger item stacks).
Since they're linking expanded backpack slots to Authenticators, it's probably that someone somewhere in the company did a cost-benefit analysis and determined that the savings from having fewer CS and fraud issues with hacked accounts would outweigh the costs of having the engineering team fix the backpack issues.
Battle for Azeroth is honestly also looking pretty feature-light as far as expansions go, so they might have more wiggle room to address other long-standing issues with the codebase.