Fundamentally, Reddit is the exact same sort of website as Hacker News, but Hacker News seems to be operating fine. What are these unavoidable AWS-hosted services you're thinking of? I cannot think of any but I'm sure you know of several.
If you're running RightScale, you're relying on AWS. Parts of New Relic. An Amazon affiliate service.
More generally, other tools such as a CDN, DNS, email delivery services, Facebook authentication, Twitter feeds, OpenStreetMap. Google APIs, mapping, Analytics, and others, all rely on some third-party infrastructure, if not AWS itself. I'm not saying that all sites rely on all of these, but many use one or more.
The Web is becoming increasing interconnected in ways that increase fragility.
