Current FCRs and SAMs may not work at long wave lengths, but what is top stop piecemeal upgrades of an IADS to embed long-wavelength capable assets?
Not with SARH. The problem is that at 10cm on up, you can no longer fit an antenna into the missile for physical reasons. As such, without ARH a longwave radar cannot provide illumination for a missile.
Furthermore, the accuracy of radar decreases with the frequency. At long wavelengths, the accuracy, even if it were possible to make a longwave SARH seeker, would be insufficient to provide guidance, even with proximity-fuzed weapons.
Take AEGIS for example, or reverting Patriot back to proximity detonation versus hit-to-kill. Any SAM system using mid-course guidance could (in theory) have the addition of a proximity/detonation command and coupling to a long wavelength radar.
Note that PAC-2GEM+ is still in service, and 48N6 is proximity fuzed.
Even the US would likely employ waves of cruise missiles to degrade the IADS first
This is getting more expensive as time goes on. 1x S-300PMU2 would likely be able to handle up to 40, and possibly up to 50, incoming missiles. If deployed in overlapping regions of 3, 150x TLAM would be needed to suppress one portion of an IADS, not considering SHORAD.
I start with things like ITALD/MALD with stand-off jammers covering an inbound wave of tomahawks with SEAD aircraft overlapping to take down the FCRs.
Command's modelling of ECM effectiveness is rather optimistic. With the advent of new AESA and bistatic radars, the ability of jammers to degrade or eliminate SAM effectiveness is reduced dramatically, even with platforms as sophisticated as the EA-18G.
Edit:
That is the crux of the economic viability I was bringing up. What is the relative cost of the IADS components and the upgrade paths to defend against the F35 and its peers relative to the aircraft themselves?
F-35 is actually reasonably priced in comparison to its competition. Current LRIP 8 costs are 110$ mil/aircraft, with FRP costs somewhere between $80 and $90 million. Dassault Rafale is $110mil FRP and Eurofighter tranche 3 is $124mil. The only real competitor that's less is Gripen at about $70 million, but Gripen is substantially less capable.
Furthermore, IADS are very expensive. 1x S-300PMU2 is around $300 million, and the radars and SHORAD increase that cost substantially more. The new generation of AESAs are also not cheap, though prices are not known at this time.