When is Singleton actually justified?

Singleton is justified only when having a single instance is logically necessary and does not break the modularity of the system.

#Global system resources

For example, a logger, a configuration manager, a database connection driver, a thread pool. Here there really should be one object managing shared state.

#Hardware or external interfaces

When interacting with external devices (printer, port, sensor), it is important to have a single access point to avoid conflicts on simultaneous access.

#Infrastructure services

Caching systems, routing, event buses, task schedulers - anything that keeps the rest of the logic running and does not depend on business context.

#When the lifecycle is easy to control

If the application is small and Singleton does not create testing problems (for example, in desktop or embedded scenarios), using it simplifies the structure.

Conclusion: Singleton is justified when the uniqueness of the object is inherent to the nature of the task and when alternative solutions (DI, factories, contexts) would be excessive. In other cases it is better to avoid it, so as not to create hidden dependencies and tight coupling.