I’d rather have sluggish UI with proper feedback than potentially inconsistent states which I often experience with modern SPAs. At least that represents reality. Just today I was posting an ad on the local classifieds page, and the visual (client) state convinced me that everything was fast and my photos are uploaded. Turned out all state was lost and never reached the server, and I had to redo everything again.
Fortunately every browser made in the last 25 years supports keepalive. e.g. Firefox (and according the the reporter of this bug, Chrome) won't even let you disable it[0].
Phoenix Liveview works pretty good without client side state. Sure if you just have a toggle mobile menu you might sprinkle some JS for it but other state lives in the server and the delta is sent to the client via websockets