What is the difference between an unstable and a stable version? Testing by users. So as soon as enough users have tested / enough time has passed without issues, an unstable release becomes stable. In the best case nothing about the code needs to change, the release just needs to be promoted to the stable channel / dist-tag. That is pretty common practice for a lot of software and especially packages on npm.

With dist-tags, you can still make the versions meaningful. Between unstable versions, you can still express in semver terms what type of release it is (patch/minor/major). In classic prereleases, that is not possible. There is no semantic relationship expressed between a -alpha.1 and -alpha.2. Was it a bugfix? Does it add a feature? You don't know.

The issue is that npm did not mention in their blog post that 5.7 was released on the unstable `next` channel. The issue is not release channels in general.