Are there reasons to go back to npm? I switched back when yarn came out and haven't looked back. Been super happy with yarn. Can't say the same about npm.
People are more likely to already have npm installed and to be familiar with it. So there's an argument to be made that all else being equal, picking npm lowers the barrier to entry for new contributors. This consideration could be especially important for open source projects.
That's a valid point, but I don't think the barrier is particularly high. I've done the switch from npm to yarn once. It was a process measured in hours to understand the differences. It's not like Git vs Subversion or something like that.
I don't think that's very compelling, versioning-wise (it's still independently versioned). Futhermore, the official node docker images come with yarn pre-installed, and there appears to be no way to bundle in a specific npm version in source control, like you can with `yarn policies set-version` (v1). That has worked wonders for us. Before yarn we used to have problems with developers using different versions of npm on their machines/build agents, and .nvmrc/"engines" doesn't help you there other than being an "error gate". The yarn executable acting like a shim delegating to the checked-in version is brilliant for versioning (especially CI).