As I see it, the main issue with new syntaxes is the fact that they break compatibility with existing tooling. JSX is not fundamentally different in this regard, except for the fact that it gained so much mindshare that it brute-forced its way to getting first-class support in editors, language servers, different frameworks, even TypeScript. I don't predict that very many new JS-adjacent syntaxes will get that treatment from the industry.

True, although Svelte's syntax is chosen cleverly in a way that is not incompatible with existing tooling. (Starting a statement with $: is syntactically legal JS and a no-op.)

Another question is how much of Vercel's resources would it take to contribute Svelte support into all the major tools.

It's not technically incompatible, but it's mostly parseable nonsense in the plain language. So it really only serves to make sure syntax highlighters still work, which is just one of many aspects of tooling

Considering JSX's current level of support involved cooperation from multiple FAAMNGs, I doubt Vercel could match it

Can you give an example of tooling that is not compatible with encountering the $: label? I can only think of a linter complaining about an unused label "$" or a redefined label "$".

Compare this e.g. with how React linting has to know which functions are actually (custom) hooks, and they cannot have any idea about the semantics of custom hooks.

I'm sure it will let you make that label, but it won't be able to make any sense of it. Maybe I was slightly wrong that only syntax highlighting will be helped by the "standard" syntax, but the point remains that none of the (existing) tooling can properly make sense of those parts of the code. That $ isn't really a label, it's a reaction, which your linter knows nothing about and can't help you with; at best it can "work around" it.