SharePoint is already legacy. It was built in a world that needed a better enterprise solution for basic document management capabilities than the big enterprise content management ECM vendors were offering. And it spread like wildfire because it was easier to deploy and was more end-user focused than the large ECM tools.
It was laughed off by the ECM tools because of all of the functionality that it lacked. But the lack of functionality was exactly what made SharePoint so dangerous. It provided document management functionality that was good enough for end-users and IT with a much lower cost of deployment.