To address the real-world problem of the decline of small and medium-sized merchants and platform ecosystem imbalance caused by rising traffic costs, this study draws on platform ecosystem theory to develop a multi-agent coopetition simulation model involving the platform, leading merchants, and small and medium-sized merchants. It examines how traffic allocation strategies affect platform profits and ecosystem health, as well as the underlying mechanisms, and validates the findings through real-world business cases. The results indicate that increasing the profit orientation of traffic allocation strategies may enhance platform profits; however, it deteriorates ecosystem health, inflates average product prices, and causes user attrition, indirectly undermining profitability. Furthermore, platform traffic monetization exerts a dual-edged effect on small and medium-sized merchants. Maintaining an appropriate proportion of organic traffic can safeguard the participation of small and medium-sized merchants while intensifying paid-traffic competition can increase platform revenue, thereby achieving synergistic optimization of profitability, ecosystem health, and social responsibility.