I've used campus daypack style backpacks, messenger bags, bonafide combat style frameless backpacks (what I call drop packs) for every day carry. I now use a top end swiss army bag that has a shoulder strap to carry messenger/attache style, as well as backpack type shoulder straps. IME, a regular messenger bag is usually not big enough, and not all that ergonomic, nor is the usual attache bag, although it is more convenient for New York style commutes. Daypack style backpacks aren't generally big enough for all that I like to carry (when you throw in that damnedable laptop brick). For real hoofing it, backpack style shoulder straps (over both shoulders) are a requisite for anything more than about a 5 lb load. For over 20 lbs on the hoof, a waist belt makes a huge difference.
The design you landed on seems like a good compromise, but methinks there may be a propensity towards overloading it, making shoulder fatigue a pretty important factor. Still, I see an awful lot of young kids using those daypack backpacks with just one strap slung over a shoulder; the bag being stuffed full of books and various and sundry items. Makes my back ache just imagining it.
As you get older, you look for the ergonomic conveniences more and more.
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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)