I was chatting with my Uncle (ex-USGS) about this a few years ago. You're going to need ~1m data to resolve a carrier's heading. You'll need something in geosynchronous orbit. If your CCD is 100mpx, then you need 10000 acquires to cover a 1000km-x-1000km chunk of Pacific. If you had a 1gbps(!) connection, and you used 8x32b color (which ain't great), you'll need 72 hours do download the data. Assuming the carrier is underway, it could be as much as 2000km away, by then.

So, if we had 500000 geosynchronous satellites, with a 1pbs link we could find carriers in a good fraction of the Pacific — maybe 10 or 20% of it?

If you know where it is once it’s easier to keep track of it ships can’t teleport which substantially limits the search space. Then your limit is related to bandwidth but that seems to be a case for edge compute. And I assume there are a ton of sensors that could be tailor made for the purpose, wouldn’t radar be effective?, a low resolution IR?. And there is the ability to use other sources of intelligence in conjunction. And what’s to stop adversaries building a bunch of neutrino detectors which nuclear reactions cannot hide from, currently they’re slow to detect but it’s probably easier to fix that by building a ton of them. I suspect above ground neutrino detection technology is now good enough that stealth and nuclear are no longer compatible, and that includes nuclear powered submarines.

1) compression massively reduces that data requirement. 2) relatively simple processing on the satellites itself can cut down most of that data by only downlinking interesting observations. 3) If you're looking to target a carrier with a missile you also only have to look where you can actually deploy that missile to drastically cutting down the area you'd need to search.

Modern CWIS are for drones both sea and air and slower cruise missiles. They would be useless against hypersonic missiles.

They're useless against all that other stuff too. The operational history of CIWS consists exclusively of friendly fire.