It starts as simple crud, but then product introduces a business rule where some fields need to be hidden when another option is selected somewhere. O, but not when this checked, etc. Having a reactive framework when this happens is very much needed to keep all these effects working without a lot of vanilla JS.

This can be solved later, but is a lot harder when you have an entire application that needs to bee kept working, and almost trivial if you started with some reactivity built in.

As I said, any app with at least a "moderately complex UI". If your app is just a sequence of web forms and stays that way forever, then you're not the target audience of a UI framework anyway.