There is probably a reason they did with a ground-up rebuild and partially this is because svelte 4 didn't enable universal reactivity and $ was more confusing

There is probably a reason they did with a ground-up rebuild and partially this is because svelte 4 didn't enable universal reactivity and $ was more confusing than useful. Furthermore <slot /> had limitations in terms of children which made it difficult to build UI libraries. Svelte 5 now is actually simpler to learn for new comers despite higher verbosity. I love it!