By 2050, why will the Japanese government be concerned with elderly support ratio?

Not sure if I understand your question completely, but if I do I believe you are talking about pensions. When people pay into retirement funds they will actually receive more in return when they are retired. Money loses value over time, hence why 5 cents could get you a coke back in the day, whereas its a dollar or 2 now. So when someone pays into a retirement fund or pension, they may be paying a certain amount now, but they will receive more because of the changing value of money.