This exchange began in 2017 and the last contribution dated January 2020. I see no update or response from the HubSpot team. Have I missed something?
The issue initially brought up is one that we are also faced with. The ability to at least associate a Project task to a Sales task seems like an easy fix what with it already being possible to attach a HubSpot Asset to a given Project task.
We are working hard to centralize our collaboration within HubSpot but the fact that the Marketing and Sales sides are separate from one another is silly. As soon as you have access to both, the task management function should be integrated.