I suppose a back pack where the design allows air to flow between the bag and your back for cooling would be nice. These back packs exist. But Never been a fan of back packs. They're just uncomfortably too hot on my back especially when going a bit faster. Probably OK if you're only riding a leisurely pace and live in cold climates or riding during cold seasons.

I still prefer rack and pannier bag. I don't care how others say how ugly it looks. Having nothing on my back helps keeps me cool and comfortable even when I'm in a hurry and riding as fast as I can. Panniers have little aero penalty as others think it might have. The panniers are already being "sheltered" by your legs from the wind. It's ugly but if you can tolerate ugly, it's the faster and more comfortable option.

I once used a backpack regularly but I simply tied it on my rear rack with bungee cord. Never worn it while riding.