Population decline may not, in the long run, be a bad thing. The only downside that I see is that the young people will not be contributing enough to keep the Ponzi schemes known as Social Security and Medicare going without taking away from some other “social programs”, which may have to be rethought.
People will just have to keep on working well into what would have been retirement years, essentially working up to the day they die, if they have not made provisions for a continued cash flow from other than wages.
People who keep working tend to live longer. One of the little secrets about retirement.