As an FYI, crates.io team member here, I worked for npm for 3 years. I have been and intend to continue sharing the best practices I learned from that team. If you have any questions, feel free to direct them to me.
As for why name spaces were originally introduced, it was to help create a paid product, private packages. Very few people used them, until around 2 years later when we introduced another paid product, organizations. Free and paid organizations are the primary use case that folks use and it’s largely as a result of the increased and more ergonomic permissions controls. crates.io already affords this by letting folks leverage GitHub teams. So strictly from a npm point of view, crates.io does not currently have a motivation for this feature as we don’t need to make money and already give folks the ability to organize permissions via GitHub organizations.