Why a 500-ship IJN escort program was unrealistic

Was a program of 300 Type C and 200 Type D IJN escort ships realistic?

No; five hundred escorts was an aspirational figure beyond Japan's practical capacity.

The proposed total was beyond realistic Imperial Japanese Navy capacity. Five hundred escorts would require far more than hull steel and simple machinery. Japan would also need crews, weapons, yards, fuel, maintenance, and an organization able to deploy the ships across its sea lanes. The aspiration therefore exceeds the industrial and institutional resources available while Japan was also sustaining its combat fleet and merchant-shipping war.