It is completely hostile to users. If I close a web page I expect everything related to that web page to stop. Immediately. I don't expect there to be lingering background threads consuming battery life, network data and disk storage. And in Chrome you can only discover service workers by enabling a Debug mode and memorising a specific URL. Exactly how does a non-developer figure out what is going on ?
If Apple had any sense they would ban service workers and instead propose use case specific, tightly focused and managed APIs that focus on security and battery life first.
Also just a hint at a future where Botnets are running in your browser:
https://sakurity.com/blog/2016/12/10/serviceworker_botnet.ht...
Think about it this way... why do you have to download an entire app just to use a site? Don't like the notifications? Turn the off.
Again, the concept of background tasks isn't necessarily a bad thing. But it needs to be locked down to specific use cases not a free for all allowing arbitrary JS to be executed.
[1]: https://developer.apple.com/notifications/safari-push-notifi...
AFAIK Service workers are still under consideration, and nothing was announced.