Are you saying the population level now is perfect for social and economic development, or that increasing population is essential to economic prosperity and social advance?
I'm saying that maintaining a relatively stable to growing population is vital for economic development. You cannot maintain economic viability with an aging population. It puts too much strain on the healthcare industry, on social services, etc...
Nevermind the fact that you have fewer people to fill the same number of jobs. There will be correction with time for the jobs issues (some businesses will bite the dust inevitably), but again, how do you maintain a country on a fixed income with an ever-growing need of healthcare and a dwindling number of healthy, young individuals to pay for it?