Wall piers are widely used to enhance lateral stability in bridges with tall piers and relatively narrow decks. For this type of structure, the longitudinal direction of the bridge is commonly acknowledged as the governing direction for seismic performance, with wall piers serving as the seismic critical members. However, progressive scouring reduces foundation strength and stiffness, leading to increased transverse seismic deformation. This deformation amplifies the risk of pile damage and may shift the seismic critical member to the foundation. This study investigates the seismic performance of wall pier bents in bridges, explicitly focusing on the effects of riverbed scouring. Through a Taiwan-based case study, seismic performance is evaluated at various scour depths, identifying the seismic critical member through capacity spectrum analysis and the peak ground acceleration corresponding to the performance limit of the wall pier bent. The findings highlight that seismic performance is frequently controlled by the transverse direction, emphasizing the foundation as the seismic critical member. The effect of employing foundation strengthening as a retrofit strategy is also assessed, revealing that it provides only limited improvement in seismic performance. Even after retrofit, the seismic performance of wall pier bents remains primarily governed by the pile foundation.