You must be against illegal immigration.
Its one of the factors that drives growth of the GNP in the USA.
There are many other economic factors that are used to judge an economy. GNP growth is just the easiest for the mainstream media to understand and pass through to a public that has little training in economics.
A decreasing population can be very bad on a macro and/or micro economic level...But if properly managed, and done so at a slow rate of shrinkage, it can actually increase the wealth of a nation on both a macro and micro economic scale. It just isn't something that is easily sold by politicians who tend to be "Progressive" and want to promote grand visions of growth and prosperity. (Ie. urban sprawl, busy motorways, smoking factories, dams, bridges, etc...)