The USAF, USMC and US Navy are all short of F-35 qualified maintenance personnels. Upgrades (retrofit) can only be fielded on that level of airframe number by the private sector. That would raise the hourly cost of F-35 qualified technicians, accelerate turnovers and compete with available assets, ultimately raising cost and delaying manufacturing for the military.The two largest buyers - USAF and International partners - would disagree. The former wants the upgraded performance and weapons that are promised with block 4, and the latter wants its own weapons integrated on the platform. All customers want to avoid having to buy TR-2 hardware and then pay millions down the road in upgrade costs. The urgency to upgrade is partly because of the want for to avoid having to integrate weapons badly needed now and in the near term with the older hardware. LRASM/JASSM comes with Block 4 but imagine if you also had to integrate this with TR-2 hardware.I am not sure that there is such urgency for the upgrade. I am pretty sure that current F-35 match the current threat.
I would be pretty satisfied if assets get focused mainly into increasing manufacturing outputs. Then, they probably have a decade for the move.
It's what anyone get outstretching a finite ressource.