Germany's population essentially hasn't increased in 40 years. Their nominal GDP has increased by 15 fold in that time (even after accounting for inflation it's a significant expansion, all due to productivity gains).
Well, it probably will if a large number of people leave and that doesn't have to be bad news for the ones that stay. Of course, that will depend on the specific situation. I can't come up with an example where there was a large decline in population in an area on a voluntariy basis.
In countries with declining populations, ought the economy to contract?