I don't think NAVAIR, nor the USMC, have any interest in increasing the number of TMS under management, nor any interest in adding to their costs and problems with the management of another configuration of the Harrier.
The "birds as parts bins" is the most likely outcome, based on both management and economic perspectives. Glad to see the airworthiness issue, and cost, raised up there. Another reason NOT to do other than use them as parts bins.
How worn out the F-18D is compared to the AV-8B is a red herring.