App UIs should be optimized for their use case, yes, but I would argue that the idea that a one-off app UI design can do common widgets and interactions as well or better than a platform with tens of thousands of dollars and man-hours poured into user research involves just as much or more hubris as thinking that there’s One True User Interface. The average app with bespoke UI is chock full of usability/accessibility issues that wouldn’t be there had they been built with a platform UI framework.