Abstract The realization that evolutionary feedbacks need to be considered to fully grasp ecological dynamics has sparked recent interest in the effect of evolution on community properties like coexistence and productivity. However, modern coexistence theory being limited to pairwise interactions, little is known about coevolution and diversification in rich communities. Here we study such properties in a general trait-based model of competition on a niche axis. We leverage the structural approach to show that the effects of coevolution on coexistence are two-fold. On the short-term our results show synergies emerging between productivity and coexistence, while on the long-term diversification and niche-packing destabilize communities, thus inducing a long-term trade-off between productivity and coexistence. In light of classical and recent work, our findings help advance understanding of evolutionary effects in high-dimensional systems. We illustrate how our theoretical predictions echo in observed empirical patterns. Finally, we discuss their implications and provide testable hypotheses.