The design strongly relies on the strength of the 4, 3 1/2" oak rails that span the 90+ inches. The method used to test for SAG shown below is tried and true for me. I believe that there should be no noticeable future sag.
Below is the test fit on top of the desk. I still have quite a bit to do, add doors, cabinet backs, varnish on the outer surface of the front and back face frame, add feet to secure the unit to the desk top, a cubby unit to house 9 in/out drawers in the middle, the center arch trim to match the doors, and the top trim.