I beg to differ. Tried to use a kaios nokia 800 tough. Firefox/KaiOS using a stupid kind of webview for every UI, you get two consequences: first, everything is ugly: wrong padding everywhere, complex layout with non-integer number of list items fitting on screen (this is important, vertical list is the primary widget on small screen dumbphones). And second: everything is sluggish. My true "series 30" nokia launches app instantly, whereas i'm sure the SoC is order of magnitudes slower.
So the UX definitely shows complete absence of culture of design in this kind of space (eg gameboy UI design, classic dumbphone UI, ...). Small screens need very simple widgets (most importantly grid and vertical list), hand made paddings and proper space optimizations, most likely some bitmap font tailored to the resolution.. You can't just slap some dynamic layout and expect it to work nicely on such a small screen. Everything vectorized will be both costly and of dubious rendering quality, etc..
And of course these things only have "decent", but definitely not "1 month scale" battery life. I guess LTE is the real culprit here.
