I still don't get the seat bag thing as shown on a rigid frame. I do get it on a rear suspension frame. 200+ for a very limited use product. You could simply bungee a dry bag to a rear rack like a trunk bag for bike packing, and use the same rack for conventional touring. One system multiple applications for far less.

I do like some bike packing ideas though. I just bought a Blackburn expanding frame bag (med) which I really like and am considering a handlebar bag as well. But both of those can be added to my conventional set up or not as i wish

I honestly just don't know what a seat bag does better than a trunk bag for the price. Narrow profile for single track, more stable and weight transmitted down into the frame.