We hear this every time we deploy to a new business unit. When this comes up, I always mention that while it might seem easier to just update a spreadsheet, you then have to go and email everyone on the time, update status documents for clients, and then update resourcing spreadsheet for resource manager. And maybe you have to have a meeting to explain everything to everyone.
But if you make the change in Workfront, people already get notified of changes, reports are automatically updated, and resource grids/capacity planning tables are updated. And no meeting!
The big thing is to talk about how the information is being used from the projects in Workfront so that they see it isn't about creating more work for them as a PM but helping the agency as a whole operate.