For a large code base, this is a massive step backwards. Open up a Svelte file and try to figure out which, if any of these, are reactive:

  <script>
  let thing_a = createThingA()
  let thing_b = createThingB()
  </script>

There's no hints from the Svelte language. There's no hints from the tooling. You have to manually open up both of those functions to see what they're doing. For a large code base, they probably just call another layer of utility functions so you that's another level to dig deeper.

And that's on top of plain *.js/*.ts suddenly being hi-jacked. New team members can look at the *.svelte extension and know to go look at Svelte documentation. Why would they think to do that for plain *.js? They already know JavaScript.