I agree that we have not yet seen a silver bullet cross-platform solution. And this has been tried before the mobile age too (Java FX, Adobe Air, MS Silverlight).
What’s important to discover is, where is the tipping point that makes native the appropriate choice? I’m not sure about the number of app users or number of features — there is no way to avoid real life experience to know how Flutter will fare in the real world.
However, even for small apps, there are trade-offs to make. For example, I was discussing Flutter with a company that wants to build an app for streaming DRM-protected video content. It would require building a plugin to use their streaming provider’s SDK to support DRM. The trade-off is whether they can live with a portion of their app code-base which will be hard/impossible for them to maintain.