I don’t see Apple ever implementing ServiceWorkers. They simply don’t want the Web to become any more powerful that it already is.
They earn tons of money by controlling the web experience on their platform. Only Webkit is allowed on iOS (which btw is crazy thinking of the fines Microsoft got just by providing their browser by default).
Somehow, Apple’s semi-fascist approach goes unpunished.
If Apple had their way, browsers didn’t exist. This way they could 100% control what’s going on on their platforms. It’s strange to think the original iPhone was conceived as being using webapps only. But nowadays Apple wants the web to be declining, and hopes some day everybody will be using native apps on their platform, so that they can control everything.