Yeah, but it’s not as though iOS is widely-used or anything. 
I’m not sure that AppCache and ServiceWorkers are really comparable, to be honest; they solve quite different problems. AppCache is about providing developers with a really powerful, hard-to-use-safely footgun – things haven’t really moved on since this 2012 article to that effect. I’m actually not sure how AppCache could be used to provide reasonable offline capabilities in a Discourse instance…
ServiceWorkers, by contrast, are a way of controlling the interaction between your app and the network in a fine-grained and completely arbitrary way.
My thinking, then, would be to build for the future – ServiceWorkers – and hope that Apple eventually works to make them available in iOS Safari (notwithstanding the fact that they would allow web apps to compete with the app store, and thus go against Apple’s financial interests). This would bring benefits to all users of Chrome, Firefox, and Edge, regardless of device. I’m not sure what the stats are like around the world, but that would bring decent capabilities to 60-75% of our customers.
And Progressive Web Apps are all about Progressive Enhancement, after all. 