I think it's absurd to say that "developers can press Cmd+\, which is so good that we will accept a ~15% worse UX". The industry has very strongly decided that developer "productivity" is more important than anything else though. It's sad.
Most apps are budget constrained - doing 2x native over flutter budget doesn't get you 15% better UX. It probably doesn't even give you two working apps in the budget. And getting to them you're still likely 2x bugs and worse UX because of the time spent redeveloping common shit.
If you're not budget constrained, have a team that can execute better UX without flutter, and this matters to your app - go for native. But there's so many scenarios where it doesn't.
Dual native also gets you better portability to new platforms branching off of iOS and Android — for example most native iOS apps can be turned into suitable AR apps for visionOS with two clicks and it does most of the UI adaptations required for you. Flutter won’t be able to do that for years if ever.
It's not productivity, it's cost. If an app costs twice as much to build for "20% better UX" then that's ridiculous. You can release earlier and spend half the difference on bugfixes/polish/additional features based on what you see in the wild.