You’re missing the forest for the trees… Skype is a corporate product run and owned by a single entity that is free to change their tech however they please. So

You’re missing the forest for the trees… Skype is a corporate product run and owned by a single entity that is free to change their tech however they please. Something like email, a federated and distributed communication protocol, is incredibly slow to change because the stakeholders are numerous and the protocol is entrenched. You point to DHT as a game changer, and yet that protocol was invented in 2005 and took over a decade to hit mainstream success and hasn’t changed much in the past decade. New protocols can be invented at any time but getting them adopted is a Herculean task with little monetary profit and updating said protocol in any innovative way is nigh impossible.