ESRI UC introduced a new product called Experience Builder ( ArcGIS Experience Builder | A new way of building web apps) which eventually will replace Web App builder. The new product will be based on JS API 4.x, React JS and Typescript. So, no more DOJO and Web Appbuilder will never be upgraded to JS API 4.x
The presenter said there are no plans to replace Web Appbuilder YET. We have seen it before when ESRI announced Web Appbuilder with no plans to discontinue support for FLEX. A year later they did.