In explaining the motivation, the authors of the proposal including Gil Tayar, Daniel Rosenwasser of Microsoft, Romulo Cintra of Igalia, and Rob Palmer of Bloomberg wrote that, during the past decade, the case for static type checking has been proven out fairly successfully.
Some bundlers have done both. Over time, it is anticipated there will be less need to downlevel compile. Eventually, the only step for many TypeScript users between writing and running code will be to erase type annotations. The proposal will reduce the need to have a build step. Users will simply run the code they wrote.