[Klathmon]

And it wasn't all just "strongly held beliefs" there were very real problems with upgrading parts of npm.

npm for a long time was built to do what npm did. There was no spec, no "rules" it did what it did, and changing that behavior was a breaking change. Everyone could agree that feature X needed to be fixed or redone, but doing so would break a significant number of packages/projects so it wasn't done.

Yarn was the solution, they could start from scratch, not worry about those older/undocumented/arcane edge-cases. They didn't have to care about backwards compatibility, or even reimplement the same API.

Every time this question is asked, I also like to point out that many of the Yarn devs were npm devs, and the project as a whole not only had the "approval" of npm, but also was in-part encouraged by them.

Competition is good, and it's "the javascript way" to have multiple competing tools that each prioritise different things. Yarn is pretty much a perfect example of that.