The U.S. is attempting to build satellites that can track a hypersonic missile. I don’t think it can be assumed that mildly supersonic targets can also be tracked,
I'm not so sure, given the USAF and USN seemed confident enough in tracking Tu-22M regiments with DSP for strategic warning. That said, this is mostly concerning tactical fighters being able to engage a VLO target at BVR ranges, and supercruise resulting in a higher signature.
even assuming the Chinese have sufficient technology and launch cadence to mimic the U.S. system.
Assuming anything less is a bit silly, and there's no serious harm in overestimating the capabilities of an opponent, either.
Also you still have to realtime that data to you fighters for it to be of much value.
I don't think orbital tracking is a serious threat in the near term, but being tracked by PIRATE or some similar system, and attacked from BVR ranges completely passively is going to be a headache. That's going to throw a wrench in mission planning, for one thing, since it's much harder to estimate the relative capacities of IRST compared to radar, and it's going to give aircrews less options for handling threats.
I would still want the capability to supercruise. It lowers transit times and imparts energy to weapons without reheating, and afterburners without a doubt are a big IR detection hazard. The more moderate skin heating and dry thrust signature is going to be detectable at shorter ranges.
Supercruise isn't bad, but it isn't useful within the context of a Pacific War, either.
Selex says they can use skin heating alone, above 300 kts, as a detection measure with Skyward-G. PIRATE is claimed to be able to detect targets at 90 km, and provide VID at 40 km, and presumably Skyward-G is more capable than this. Manufacturer claims aside, IRSTs have generally improved enough that they are a viable sensor for counter-VLO in the air defense role, which is going to be the real issue.
Will be able to detect low-radar-cross-section targets, company claims
aviationweek.com
Supercruising won't help with this problem of "passive BVR engagement by air defense fighters", but maybe some sort of active infrared stealth or thermoelectric skin could do it, though that's probably for "7th generation" fighters to figure out.
ACE will probably give NGAD good loiter time with a supercruise capability on top of that, which is rather unlike F119, at least.