The success of JSX has proved that the second curly is unnecessary. Moreover, a lot of people — particularly those who have been exposed to React — have a visceral negative reaction to double curlies, many of them assuming that it brings with it all the limitations of crusty old languages like Mustache and Handlebars, where you can't use arbitrary JavaScript in expressions.

IMO svelte does have a responsibility to teach/demo the basics of "functional javascript" probably as a docs/tutorial/demos chapter on "the power of javascript expressions"

I could imagine people putting a more complex expression in an @const than we typically find in svelte expressions today, which might create more demand for those blocks to have TypeScript support, which I don't think they have now.