I think if it wasn’t for the UCI regulations we would see a lot more high-end road bikes today without seatstays or traditional front triangles. We see more design variation in tri-bikes (eg Cervelo P3X) for this reason.

Full length, dropped or no seat stays are all viable structural design solutions. Torsional rigidity should not be a concern with any of these choices if implemented properly. The typical dropped stays we see on the current crop of world tour road bikes are all plenty stiff enough.

Full length stays are clearly at an aero disadvantage, which is why dropped stays are now pretty much the standard for all aero road bikes. I don’t believe there is any compromise in dropping the seat stays other than subjective aesthetics.