Classic also supports really powerful programmability, such as forms and custom views, and although I accept that the golden age of MAPI has long passed, its a shame that nothing in New comes close.
However, that speaks to Microsoft and their vision for what they call Modern Work. In this vision, it is Teams that is the center of the universe. Teams is the thing that gets the platform treatment with plug-ins and apps and polish. Outlook is relegated to something the old business people use, and as the generations shift from writing long form messages to thinking anything over a paragraph is tl;dr, I can see Outlook dying off in a few years.