Yeah it's very aggressively cached for read purposes. You could write most Q&A pages to static html files if you felt like it. Stack has a similar age-toward-zero outcome that Wikipedia has (the % of new content as a total share of all content on the service chases toward zero over time); the more you fill out the site, the balance of aged content to new content inverts to an extreme, so mostly what you end up doing is serving up cached reads on older content.
They're fairly well over-provisioned on server capacity. It's so relatively light to operate on the infrastructure side that they could safely further reduce the server count. There's no need in their case however, since infrastructure cost isn't one of their big problems.