The OOB field "Est, Revenue" should be made read-only by enabling this "Field is read only" check box on the field and have the "Revenue" field set to "User Provided.

I understand that If you select System Calculated, the Opportunity Lines’ Total Amount will add up to the Est. Revenue field and the field will be locked for you. If you select User Provided, the Est. Revenue field will be unlocked with whatever value it already has and you can change the same.

However, we expect an OOB way to make this field Read only irrespective of the Revenue field