The reason for the decline is that people are living longer and immigration. A socialist structure is designed as a type of ponzi scheme where the first people in tend to fair a lot better. The more humans that come in and are dependent the system the harder it gets to maintain it. This causes a cycle where people will think the only real solution is to increase socialist government programs. Ultimately this causes things to get even worse until things just cannot function anymore.

The ponzi scheme will collapse when the value that citizens provide to society is less than the value needed to maintain the ponzi. All socialist programs like welfare and healthcare are a slow burn towards economic collapse.

Theoretically if the people in charge of maintaining the ponzi are super careful and never make any mistakes then socialism would be vastly superior to other options. The problem is, humans make mistakes and ponzi schemes are merely just a means to an end for profit.