But, doesn't maintaining both implementation complexity and product usage complexity low require either more development time or better developers?

Reducing either form of complexity certainly costs resources, but I think the point is that they are not necessarily antagonistic goals- they might even come as a package deal, to some degree.

There's a whole spectrum of users, as well. Programmers can be Python users, wherein the python implementation takes complexity hits to simplify it for the user.

It's all subjective, and the design of the high level concepts (maybe mechanics?) influence the complexity at every level below it. The idea of a 'file' goes from the GUI down to the hard disc format.