.....over 30 years for me!
Backpacks are better as you carry more weight for longer durations and/or distance. Their disadvantage is that you have to take them off & back on everytime you want to access your gear. Shoulder bags are better for less gear and more frequent access. Their disadvantage is that they're less securely attached (ie you stumble and the shoulder bag is off) and they can MUCH more quickly tire you (your shoulder). I've had shoulder bags occasionally but usually get rid of 'em, reverting to backpacks. When I discovered Lowepro Slingshot 200 a couple of years ago I've used it exclusively except for long mtn hikes where other non photo gear is also needed (ie water, clothing, food, 1st aid, etc), using a backpack. Slingshots carry your gear largely on your back via a hip/waist pad, a shoulder strap, and a stabilizer strap that readily unclips to allow quickly pivoting around to your front and allowing not only access to your gear but also a shelving area for handling multiple items during lens etc exchanges.
Give into the reality that you'll own/need multiple bags. The earlier you concede that there is no such things as the universal perfect bag, you can then start to focus on getting the best small array of bags to cover all of your needs.
Well said and I was one of those persons that was looking for that mystery perfect bag and am content with having a shoulder bag and backpack. It depends on the shoot then I decide which bag to grab.