I think desktop support will always be there in some form or another because I believe to port flutter to some particular platform wouldn't be a massive amount of work. Reason is that Flutter is written to address a graphics layer such as OpenGL or equivalent. The iOS/Android apps it generates are just a single view with graphics and touch input, similar to a Unity game for example.

They used this to create presentation app during last Flutter Live, so it seems strange that it's still "exploratory effort".