The number of developers defending this decision may also partly explain the decision. Developers' skill has many dimensions that are not perfectly correlated.

The number of developers defending this decision may also partly explain the decision. Developers' skill has many dimensions that are not perfectly correlated. It's quite possible that even among the (otherwise highly skilled) python core developers, few understand (or care) how API design affects reliability of code written by the library users. And the few that do, have hundreds of higher priority issues to deal with.