Being an admin user, currently we can see list of all WS Projects whether those are shared or not shared with admin user account. From this list if admin wants to check the latest usage of those projects then only option is 'Last Open' data. But admin will be able to see this data only for those projects which are opened by his/her account and not by other users/owners. And will not be able to calculate the actual usage of all projects/dashboards.
If adobe can provide 'last opened' details to Admin for all projects opened by any user then admin can contact those users and remove the unused projects/dashboards.
Hope I am able to highlight my point OR please let me know If need more details/discussions from my side.