A workspace is rarely a solo effort. A team manages the screens together: someone owns the content, someone keeps the schedules right, someone watches that everything stays online.
Accounts are the people; the workspace is where they come together. Several people, each with their own account, can work in the same workspace at once.
Access lives at the workspace level. You do not hand someone a single screen or one playlist on its own. You bring them into the workspace, and what they are allowed to do applies there. One person might manage everything, while another only looks after billing and never touches the screens.
This is why the account and the workspace stay separate. One person can belong to several workspaces, and a workspace can hold many people, without either being locked to the other. The account is the person; the workspace is the work and who is allowed near it.