Very much agree with José - we have limited time and I would prefer to spend that on things I enjoy or find will yield the best value.
For me the backbone of an app is of extreme important - that’s why I would prefer to use Elixir and Phoenix on the backend.
After that, for the kind of app I want to create (a social network), the native apps are also hugely important - so I would prefer to spend my time learning Swift (and the Android equivalent).
Since these kind of apps will predominantly be used on mobile, the web version is less important (to me) because as discussed previously, native apps have considerable benefits - such as access to specialised hardware which can help differentiate apps from the competition to give them an edge 
So my time:
- Learn Elixir and Phoenix
- Learn native iOS (and possibly Kotlin or an easier route to Android - the Android app doesn’t need to be native as the platform does not have the same kind of exciting and unique hardware that iOS hardware does imo)
- Build a web version but as more of a secondary (probably less-feature-rich) platform. LiveView will mean I don’t have to become proficient in another language and learn another framework or set of tools. It will also mean it will be quick and easy to prototype new features.
But each to their own 
Personally I am very excited to see the kind of apps people will create with LV and I’m fairly sure many of the doubts and concerns will just fade away as the technology proves itself (which I have no doubt it will with Chris and José at the helm!!) 
