They mentioned that the team tried around 50 different variations. Meaning they spent a lot more time than a comment on this. I would trust they know what they are doing considering track record of Svelte team.
Tbh this seems kinda rude. Maybe check github i am sure the whole journey can be seen there.
I think the reason they didn't opt for this approach is that putting stuff in a {} block creates its own scope, so any variables declared within that block are inaccessible from outside said scope.
The $: label syntax is valid JavaScript, but allowing scoped variables to be accessible outside of scope is very invalid JavaScript.
I'm just brainstorming here, but what about an optional $reactive rune where everything in that is reactive by default?
$reactive(() => { let name = 'world'; });
vs
let name = $state('world');