It's telling that Service Workers and Web App Manifest remain unimplemented by Apple but they are releasing a (proprietary) payments API, as though it's a bigger need.
Service Workers from what I've seen would consume quite a significant amount of battery life and prevent Safari from sleeping under most conditions.
Service workers are nice but nothing earth-shattering, there are more interesting features in development at the moment: https://webkit.org/status.
According to the list they are also considering implementing service workers so it is not all drama.