Currently minimum-stability is set to "dev".
I think we should rely on composer defaults and have the minimum stability set to "stable" just by removing that line.
https://getcomposer.org/doc/04-schema.md#minimum-stability

Then we will need to add dev, alpha, beta, RC suffixes to the specific dependencies that need it like for example devel or drupal console.
https://getcomposer.org/doc/04-schema.md#package-links
The composer.json get also more semantic.

This way we don't hide dependencies on modules, libraries, etc that are not so mature.

This all started for me because there is a contrib module with a broken HEAD
https://www.drupal.org/node/2872222

What do you think?