There are absolutely reasons where a native app is worth it - I just don't think building your own backend or not factors into that decision much.

Maybe the point you are trying to make, is when you have enough resources/large enough company, having duplicate teams for each native app isn't that big of a deal? I agree financially, although is is harder to technically coordinate two teams with dual releases and implementing features twice, with twice the bugs, and it slows things down. (Maybe not a big deal to Bitwarden - their app featureset may be quite stable, IDK)

(Disclaimer - I've been on teams building kotlin/swift apps and also cross platform apps professionally, so this is my firsthand anecdotal experience.)