I'm a woman with small hands and unfortunately smaller pockets. I don't need a thinner phone - I need a shorter, narrower phone that's comfortable to hold and that won't peek out halfway from my jeans.
So for me, the flip phone format makes a lot of sense, because companies seem intent on offering me Large or XXL Max Extra only in terms of candybar phones.
That said, I don't think it should be the mainstream, never mind the silly hyperbole that "every smartphone should be a flip phone". We just generally need more size/format variety in the market.
Doesn't solve the small hands ergonomics aspect, but yeah, bigger pockets would be nice. I'll take whichever quality of life improvements I can get, though.