Svelte by itself is great, but doing a complete PWA (with service workers, etc) that runs and scales on multiple devices with high quality app-like UI controls quickly gets complex. Flutter just provides much better tooling for that out of the box IMO. You are not molding a website into an app, you are just building an app. If I was building a relatively simple web app that is only meant to run on the web, then I might still prefer Svelte in some cases.