That's a cynical view, and I don't think I said you should depend on Cache-Control working. Yes, there will be bad actors, but the majority of clients are good actors. It's just one of several measures you should take to even out the load.

Of course you'd want a caching layer in front of the server doing the actual work, but it's still possible to "thundering herd" the cache server if you use an Expires header. Even if the herd doesn't hurt your backend server, it can still make the load on your caching frontend servers spike at specific time periods with every good actor refreshing the content at the same time. So it's still ideal to try and even out that load with Cache-Control.

The use case of having hourly updated data (e.g. weather data) on an S3 bucket behind a CloudFront distribution is not that niche.

Thundering herd may or may not be an issue depending on the amount of traffic you normally get, the architecture of your backend (e.g. AWS Lambda or S3 which can most likely deal with this easily) and the primary purpose of your CDN usage (e.g. caching data closer to the users for faster delivery world wide rather than reducing back end load).