With a labor-augmenting technological change, in the long run steady state, per capita variables grow at the rate of technological change, and aggregate variables grow at the rate equal to the sum of population growth and technological change. In aging economies, including Japan, where the working-age population has started decreasing, the labor force also declines, given the participation rate being held constant.
Because the scarce labor force imposes a natural constraint on labor supply, the marginal product of capital declines accordingly. As a result, macroeconomic growth would be impeded.