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.

Also Svelte is so great because developer do not need to worry about class names conflict, except of passing (global) classes to component (sic!).

You must: reference each element you are extending using refs or an id add code in your oncreate and ondestroy for each element you are extending, which could become quite a lot if you have a lot of elements needing extension (anchors, form inputs, etc.)

This is where hooks/behaviors are a good idea. They clean up your component code a lot. Also, it helps a ton since you don't get create/destroy events for elements that are inside {{#if}} and {{#each}}. That could become very burdensome to try and add/remove functionality with elements as they are added/removed within a component.

But some sort of official way to do that in the language would make this nicer - and would mean I would have to worry less about destroying components when their parent is destroyed, which I'm certainly not being vigilant about in my code.