I have been using a backpack with a rack for 30 years, the last 25 as a regular bike commuter. What really made it work for me was the bungee arrangement I worked out 22 years ago. I had been laying the bag down on its back and wrapping a bungie around it. But that took time and effort. One day I figured out that if I ran the bungies from the seat rails to the back of the rack I could just spread them and slid the bag in upright. The bungies press the bag against the seatback and provide lateral support. With 26" wheels this works great. This arrangement provides a very quick on and off and is more secure than you'd think. Somewhere I have a video of me punching the bag and shaking the bike trying to make it fall off, which it never does.

When I aquired an old roadbike I just wore the backpack. At first it was uncomfortable and tight, but I experimented and found that if I let the straps out all the way and let the bag rest on the small of my back it puts less stress on my shoulders, and allows air to flow on my back. THe front of my shoulders get a little sweatier in hot weather, but not so much my back. I don't know how the bag stays secure, but it does and doesn't sway like you'd think. I don't think this loose-lowback method would work with a more upright riding styles and of course your personal geometry and comfort will vary.

Last year I bought a new 700c bike for commuting. The seat doesn't rise as far above the rack as on the 26" bike, but my seat-rail to rack-end bungie method still works. My current backpack has grommets on the side, so I added a couple of D-Clips and snap the bungees into them for added security. This adds two seconds to mounting the backpack, and four to dismounting. Once in a while I'll get to the end of a commute to find I've forgotten to use the D-clips and there were no problems.

What I like about using a backpack on the back-rack is that it doesn't add width to the bike. On my old commuter when I deploy the folding baskets I have to be aware of the extra width, but it's really only an issue with doorways. I also like having my hands free when I wear the backpack into a store without carts, or walking to and from the bikes in my shed, and at the office.

Also, on the new commuter I held off putting on folding baskets. On a couple of occasions where I received a package at the office I just strapped that tp the rack and wore the backpack.

This is what works for me, as always, YMMV.

(found the punching video)
Also here is how I wear the backpack low: