Could you have a force mix of say 100/40 - F18/F35 ?
This would enable a potent combat force of F18s by 2020 and allow the run in of F35 as it trickles in to service.
Benefits: Earlier capability, allows run up to F35 ops, less expensive, is stealth really necessary beyond the opening days of a campaign?, preserves 'golden bullet' a/c, enables a/c redundancy, force flexibility (why take a ship load of F35s to an area when F18 is more than capable), maintenance, combat proven F18,
Negatives: Logistics, Engineering,.....