Since all node installations come with npm, probably forcing users to install yarn if they are happy with npm (since they both do the same) is not desirable and will decrease reach. It would be better if yarn could be kept as a choice since it's not the default or standar.

Fallback could be implemented as mentioned in this gist by @jsynowiec's comment:
https://gist.github.com/jsynowiec/b67221be8f34ac531f17c69fdaf1efbe