The only thing this would change is not needing to "cargo add smol" which already exists as a smaller runtime if you need it. That wouldn't solve the fact that

The only thing this would change is not needing to "cargo add smol" which already exists as a smaller runtime if you need it. That wouldn't solve the fact that a large part of the ecosystem would still need the features of Tokio, and would still use them. So you still have interoperability problems, until figuring out how to make runtimes swappable, and that is open-ended work. (and also assumes that smol's particular choices are correct for this, and that everyone maintaining the stdlib agrees that they are, and that the folks on the libs team are willing to step up and start maintaining an entire async runtime, etc etc etc)