It would indeed be downloading binaries. See the design doc, which talks about the need to set the execute bit on files in the module cache: https://go.googlesource.com/proposal/+/master/design/57001-g...
My concern is that it's not clear to me how this will look on my computer. Currently, I download the source tarball and build with the latest Go toolchain I have previously installed. I then change the path to point to the newly built toolchain.
So, in the future, what is it I'm downloading? How does the toolchain get updated exactly? Where does the new executable reside? What happens to the toolchain that I explicitly built and installed? How does Go1.21 change my path?
As I say, I like the idea in principle but there are details that I think are important, that don't seem to have been explained anywhere.