Let's put it this way, the F-35's EODAS can track ballistic missile launches from over 800 miles away. This is a statement of general, not specific capability, ballistic missile launches are extremely hot, but the long range demonstrates considerable ability. The IRST on the F-15s are a new upgrade, so they should have some comparable capability for Mach 6 missiles being .
Your point about obsolete datalinks is correct, except that there are no obsolete datalinks involved here. Both the E-2D and F-35 are new craft, as is the electronics package on the upgraded F-15.
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In general, there's five different levels of countermeasures an interceptor missile has to bypass. First, long-range general jamming. We know the US has that capability; it can be placed onto the E-2D as well as the F-15s via pods. Second, targeted jamming; a powerful radar like the one on the E-2D is powerful enough to burn out an opponent's seeker head. Third, MSDM on forward F-35s. Fourth, MSDM on escorting F-15s, which may engage close to the AEW&C or far from the AEW&C, depending on time to detect. Fifth, MSDM on the E-2D itself, since the MSDM is being designed as a sort of active chaff.
My point is that while I see the PL-15 / other interceptor missiles as a nice capability to have, they're not the most productive line of research. If engaging US aircraft before countermeasures are mature, they could do some appreciable damage, but once they become mature, their main value is instead forcing the US to throw escorts onto their soft targets, decreasing total sortie rate. As a way to defeat the USAF, they are outclassed by having better WVR AAMs, Chinese active chaff, and techniques focusing on the F-35s and F-15s instead of their E-2D.