Agree with what they said! ^^
LiveView has been making leaps and bounds, not just with performance tweaks but in areas which some might have considered weak points for this type of tech.
Have a look at things like Surface - A component-based library for Phoenix LiveView and 🍿 LiveMotion - High performance animations for Phoenix LiveView and when you really must have a SPA/heavy client based JS, then you could use Hologram - full stack isomorphic Elixir web framework alongside Phoenix! 
Personally I would go LiveView first and then (use it/Phoenix with) Hologram and then something like Svelte if I really really had to (tho in that case I would probably really be needing something WASM).
This is jmo, ultimately just go with your gut 
