Pack Mule isn't really about more space, though. It's about becoming stronger and able to carry more before being encumbered. At least that is how I look at the perk. You aren't getting any extra space with the perk. You just don't get encumbered as quickly. That is different from having extra space from a backpack.
Anyhow, that's just clarification of the perk. As far as having backpacks, I'd be fine with that. But there would need to be some balancing around that. We already have plenty of storage space by the time you have a decent vehicle and a drone. Giving us even more space basically removes inventory management from the game, and I think that's a good thing to have to worry about. I wouldn't want to go crazy with inventory management... when I use Vehicle Madness, I also use a backpack mod because of all the extra vehicle parts we need to collect. I wouldn't play that without a backpack mod because that's too much inventory management. But I think the game is at a good place for inventory management right now. So if adding backpacks, there would need to be something done to balance that extra space so that there is still a need to manage your inventory.