The mechanisms underpinning the long-term effectiveness and sustainability of soil conservation on the Chinese Loess Plateau remain poorly quantified. Here, using a physics-based distributed model (InHM) together with reconstructed representations of three governance stages (1960, 1970, and 2024), we identify a clear regime shift in watershed erosion dynamics. We show that the dominant control on sediment export shifted from a transport-controlled (hydraulic-limited) regime in 1960/1970-where yield is primarily governed by flow transport capacity-to a supply-limited regime in 2024, where sediment availability from hillslope sources becomes the limiting factor. Spatially, this transition is characterized by a migration of erosion hotspots from concentrated lower-reach deposition to dispersed upstream source interception. This shift is corroborated by a sign reversal in the relationship between sediment export and stream power (from positive in 1970 to negative in 2024), consistent with a transition from transport control to supply limitation. Consequently, the 2024 integrated system achieves markedly stronger sediment reduction (exceeding 98% under the typical storm events evaluated here) despite an approximately 66% decline in remaining check-dam system storage capacity. Our findings indicate that strengthened hillslope source control can, to a large extent, compensate for reduced gully sink capacity as check dams age, offering transferable design insight for climate-adaptive watershed management in dryland settings.