GK, there is an error margin between actual fatigue and that experienced by the testbed, hence the higher target. The fact that the F/A-18 experienced real-world fatigue issues at an earlier point than the testbed suggested is testament to this additional margin. It would be overly optimistic to expect the F-35 to meet its projected lifetime without the planned rectification.
The presumed logic of increased thrust overcoming the weight gain is a bit shaky with the F-35B due to physical limitations of the lift fan.