I want to attack the premise that a shrinking population is inherently a problem. The only clearly stated problem that I've seen is how social security will likely crumble since the fewer younger people won't be able to support the large aging population.
But this ignores how productivity has been and will continue to rise thanks to machines. I think we can easily take care of an aging population if we make proper use of new technology.
In fact I think that reducing our numbers is necessary for our survival.
We live in a capitalist world, where constant growth is assumed and expected, but the earth is a relatively closed system and you know what happens with bacteria in closed systems? They reproduce until they exhaust the system's resources and then they all die.