Can you tell me what is the number 1 feature that Svelte has over the incumbents like React?

(Not a user, just evaluated it previously. Please correct what I got wrong.) They compile the reactivity statically, so instead of tracking effects at runtime, they generate code for it. I'd guess it means slightly more JS to download, but less initialization in runtime.

However, they recently added runtime reactivity to be more flexible, so it seems to me they are becoming VueJS.

Radically simpler reactivity that doesn't require 20 different hooks to do the same thing.

Same goes for most modern frameworks (Solid, Vue, Preact) and even old ones experiencing a renaissance like Angular.