Why do you prefer to send HTML over the wire?
For one it adds a simple set of primitives I need to learn on top of HTML (a basic tech to know), so no large JS frameworks or patterns to learn.
Secondly, it's really fast with regards to development speed, application speed and just getting up to speed in general. A basic MVC framework (such as Django but others also) is all you need in addition to the primitives I talked about above. With modern frameworks such as Vue or React, I need a backend with basically the Model and Controller part and then a different framework for the View part.
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).