Agree, unless they broach 250 jets it may not be worth the investment.
It will end up being a larger program than that if the Navy meets its design and program goals. At some point in the early to mid 2030s (once F/A-XX exits low rate production), the Navy would be looking to transition from buying F-35C's to buying F/A-XX so it will be the only game in town with hundreds of Super Hornets needing replacement between then and the early 2040s. Some of the ways this won't happen...
* Navy does not secure funding to pursue F/A-XX as originally intended
* Navy does not execute well on the program severely limiting it or leading to outright cancellation
* Navy gold plates the requirements and is unable to afford to produce it at reasonable annual rates or buy a substantial inventory
* There's a breakthrough in autonomous unmanned aircraft and/or other external pressures (budgets, CONOPS etc) which forces the Navy to substantially alter its manned / unmanned fleet mix
* The Navy dramatically reduces its future carrier fleet and air wings
I think the Navy is likely to pursue a 5+ generation concept for F/A-XX. Some elements of next generation technologies mixed in with large amounts relatively proven technology baseline that's already flying on the F/A-18E/F or F-35C. The hope here being to field something that is better than the F-35C in some ways (range, payload, Space/weight/power & thermals perhaps) while being similar in other (stealth and avionics perhaps).
This approach they would hope keeps unit cost to within reasonable levels (for a post 2030 fighter) and enables them to replace at least 1/2 to 2/3 of the Rhino fleet (remaining going the unmanned way) over time without breaking the bank. This is not too different from what its also doing on the DDG (X) effort where its looking to take the current state of the art in mission systems and field it on a larger, more optimized hull with plenty of power and thermal margin for future growth. A similar path makes sense on F/A-XX. While one could say that an F-35C with AETP could meet some of that demand for increased mission system performance, power and thermals it would still be approaching maxing out the potential of the platform. A clean sheet, larger F/A-XX could start at a baseline that is similar to or better than what ann F-35C with AETP could deliver in terms of power, cooling and mission system perofrmance with plenty of margin for future growth. That, along with greater range and larger payload would justify a $10-$15 Bn RDT&E expense vs spending $2-3 Bn to re-engine the F-35C.