The challenge can come if/when your backend needs to start servicing a mobile app, or some other channel that doesn't use HTML.
But again if you know that won't happen, no need to prematurely optimize for it.
It's a totally different ballgame if the app is mostly client-side utility anyway (calculators, etc), but the HTML + a bit of JS handling Service Worker and offline presentation should enable a great offline experience. I haven't seen it done in a large scale, but it should work.
2. Or you expose an API for those mobile clients. You might still be saving more work (but this is subjective and depends on the project).