[takno]

backdrop-filter tends to go in my not-very-nice-to-have bucket. It's certainly something you can live fairly comfortably without.

Service workers are more of an issue - realistically if you want to build a serious web-app for iPhone right now you're going to have to build a shim around it for notifications and offline, although that can be quite small.

If you are aiming to provide a worthwhile experience to the users who will never install your app, or need some serious selling first then you need something on the web, so you are going to have a mobile site. Building a lightweight service-worker shim around that for iPhone and providing a couple of browser-specific stylesheets ought to be less work than full native apps.