As the fertility rate continues to decline, the number of births is falling sharply. Furthermore, the number of deaths is rapidly increasing due to aging. This study examines the future size of the population and its composition given the continuance of the fertility rate decline and the current population structure. The goal of the study is to explore the emergence of systemic dynamics over an extended time horizon. Using the system dynamics methodology, the causal feedback structures of natural population changes were analyzed. Additionally, a simulation model was developed by considering the feedback structures of delaying births (tempo effect), reducing the number of births (quantum effect), and aging. The results of the simulation model show that if the current population decline pattern and structure are left unaltered until 2100, the total population will be one-quarter of the current size, resulting in a mere 13 million people.