[csande17]

In practice, React users often end up running all their app code through a big slow compiler anyway. Usually Babel (to convert JSX to createElement calls) or TypeScript. Svelte doesn't increase the overall complexity of your build process, it just uses that compile step to do something to your code that's actually useful.

[jchw]

Adding more compilation steps is not free, though. You have to convince people to want this. It's not necessarily an easy sell.

Even though JSX is pretty React-specific, it still has managed to make its way into other frameworks and many compilers because it's fairly general at its core and because it is a fairly non-invasive process. So for example, you can compile JSX using babel, but also TypeScript, esbuild, and more. (And those are all independent implementations!)

I can't claim to know what the build process looks like for Svelte, but I have a suspicion it is far less general than the source code transformations that we have so far that provide type checking and newer ES functionality in older engines. (Which, by the way, you appear to be insinuating are not "actually useful.") Angular has a similar thing going on and it is not terribly well received.

[bayesian_horse]

Type checking and Browser compatibility are somewhat useful to some people...