There’s nothing quite as adventurous in all of IT as spending months building an application that depends on a third-party API for its success. For the most part, developers usually don’t know for sure how much stress the IT infrastructure supporting any given API can stand. If their application is wildly successful, it could suddenly slow to a crawl when that third-party API gets overwhelmed by requests.
It appears that Apple may be working on adding service workers support to WebKit. The WebKit Feature Status webpage has recently been changed from "Under Consideration" to "In Development." This change may be an indication that Apple will eventually support Progressive web apps (PWAs).