If a new version of lib2 moves to a half-baked version of lib3, I want to stay on the old version of lib2 (and lib3) until the community agrees that the lib3 change is good and everyone has migrated to it. Having two different versions of lib3 in process is begging for bewildering bugs, and it was a serious mistake for npm to let that happen (much less by default!)

While I'm not defending running multiple versions of a library, I've never seen it cause bewildering bugs, or any bugs at all. The deeply nested style of npm may have issues, but it makes it really easy to avoid thinking about dependency problems.