> because maybe in the future JS runtimes might be able to strip away types for you without a build-step.

Not just the future. Node shipped this with an "experimental flag" in the last LTS and in Current it works without the flag (will ship in the next LTS without a flag). Deno has done something like this for years now, and Bun for nearly as long. The remaining question is if/when Browser support might also exist, which for now remains at Stage 1 discussions with the technical committee (TC-39).