Having gotten "old" by some definitions, I've increasingly experienced the "friend problem". The article seemed to be addressing a "non-old" segment of the population, so I think it's appropriate to offer a few observations from a different perspective.
It's not just me of course, even my very friendly wife notices dwindling friendships despite putting a lot of energy into keeping them going. We often ask each other "why is that?", but so far neither of us has a clue.
The circle diminishes gradually, incrementally, the loss is one by one. Friends known for decades die, develop dementia, or move two thousand miles away to be near family.
Friends are people and don't always change for the better. It's puzzling and painful when long-standing friends grow "weird" and unfriendly. It happens, no way to explain it, no known illness or infirmity removes them, just another kind of loss to grasp.
Losses of such kinda are truly irreplaceable. Sure making new friends is wonderful and necessary but never quite the same. Thankfully a few stalwart friends are still around.
Experts all agree having more friends is better, thing is, starting new friendships is a constant and steep challenge in later life. We find opportunity is sparse, contemporaries hesitant to take up the offer, or distracted by a plethora of family, health, and economic demands. The experts aren't saying how to solve these issues.
If there is a key it must be the willingness to be persistent, not give up the effort to enrich the network of connectedness. Here and there people do respond, and a relationship slowly begins to form.