I weighed into this thread in 2017. That's when I was using a Timbuk2 backpack. It didn't last very long, but it wasn't expensive, either. My spouse gave me her Osprey backpack, as she wasn't using it. Wow. Now this is some serious equipment. It has a gap sort of thing to keep the pack away from my back. It touches me only on my shoulders and my hips. It's surprisingly heavy when empty, and that's because of the internal frame. The frame makes the weight sit on me so the total weight feels less than with other backpacks. It has a zillion pockets. After a couple of years of use, I just discovered more pockets. She got a women's model and it barely works for me. I'm not a large man, down to about 5'8" tall.

I've noticed that putting luggage on a heavy bike doesn't worsen the ride much but on a light bike, it makes it much worse. That's why a backpack makes sense for so many of us. It's also simpler than panniers. You can't really use panniers for anything but their intended use.