Balancing productivity and product quality remains a central challenge in biopharmaceutical manufacturing. Advances in cell line optimisation have delivered substantial gains in expression levels, yet these improvements increasingly raise concerns around product heterogeneity, stability, and consistency. This topic explores whether current cell engineering strategies are approaching biological or practical limits, where further productivity gains risk compromising critical quality attributes. It examines trade-offs between yield, robustness, and regulatory expectations, and considers whether future improvements will come from continued genetic optimisation or from complementary approaches such as process intensification, systems biology, and data-driven design. Understanding these boundaries is essential for sustainable and compliant bioprocess development.