When it comes to backpack-wearing, there are a lot of considerations other than self-defense stuff. For instance, carrying a backpack in various ways will affect your spine and bone structure. The best way ergonomically is to have one with hefty straps, with kind of a teardrop shape (most weight toward the bottom), wear both straps, and use the waist (belt) strap so that a portion of the weight is taken on your hips.
If you consider what is the likelihood that most of us will get assaulted (especially if we're aware and have good personal safety practices), and what is the likelihood that the way that we wear our backpack will make a big difference, it makes no sense to put this so-called tactical stuff before other health considerations.
I once taught a seminar at a jujitsu place where I was teaching about tactical folder use and "EDC" (everyday carry). I told them I always wear my wallet in my front pocket and asked them why they thought I did this. They all guessed it was to prevent having it "picked".
I told them that this was wrong. The real reason I wear my wallet in my hip pocket is because if it's in your butt pocket, then when you sit on it, it will be under one of your ischial tuberosities (ie. butt cheeks) and it'll make your butt uneven. Over time, this changes the way your spine aligns in relation to your pelvis. It's bad for you and you'll hurt when you're older. Not everything that I do in life is in relation to anticipating getting mugged.
Self-defense is just one part of your life, and it has to fit in with the rest of your life.