That may be the case, but as far as I am concerned, it also has one built-in advantage: everyone I know uses it.

As far as they are concerned, the advantages are the ease of use and the richness of features. As far as I am aware, there is no other single tool that combines all the features of Skype in a single product.

The whole point of a communication tool is... communication. What is the point of having a super-duper communication tool and no one to communicate with?

Lastly, the very example of Yet Another Free Tool that no one ever uses, already at least the fourth only since I joined this forum in 2012, shows how short-lived those are. All my friends, relatives and work relations have been using Skype for well over 15 years. Show me a single open-source product that has been in popular use for at least 10% of that period.