Bike porn

Compare that with a rim brake, which converts brake load almost exclusively into bending load in the middle of a long thin seat stay and you can see that pencil stays are probably more out of place on a rim braked bike than they are on a disc braked bike.

where rim brake puts the bending load is an ultra short rear bridge, which absorbs most of it and only fraction of that load transfers to the seat stays.
this is why this successful solution has not been given up on for couple of decades now.