You need to balance several factors: the need for new features, the increasing difficulty of finding support for old code, and your available time and skills, to name a few.

Changing every built-in function to accept anys would also "break" no one, but that doesn't make it a good idea. Part of TypeScript's value proposition is to catch errors; failing to catch an error is a reduction in that value and is something we have to weigh carefully against "Well maybe I meant that" cases.

I was pleasantly surprised by Svelte's templating; in the past I've found templating languages overwhelming and inflexible, but Svelte offers just the right amount of templating whilst enabling you to use JavaScript too.