Wore a backpack for 3 years, one $18 rack and pannier in Portland changed my mind
I always figured panniers were for tourers and loaded grocery runs, but after one 6 mile ride home from the office at 5pm with 14 lbs on my back and a soaked shirt, I switched and my shoulders and lower back stopped aching within a week, so what finally got you off the backpack?
I ran panniers for a while and honestly went back to a backpack, so I see it the other way. For me the load on my back never bugged me much, but the way the bike handled with weight on the rear wheel did. Felt sluggish off the line and sketchy in crosswinds, and I hated how wide the bike got for squeezing through tight spots. The backpack keeps the weight centered and the bike feeling like a bike. Maybe my rack setup was just cheap and flexy, but the tradeoff wasn't worth it for me.