> criticisms I've seen of OOP, that it too easily results in overly-complex software

With respect to other languages, many don't do OOP well ;] Indeed in the OP video Alan Kay said "Object oriented programming never made it outside of Xerox Parc - only the term did" (1:06:40)

> Smalltalk encourages an explosion of complexity

Smalltalk is a fantastic prototyping language. Its easy to explore a new domain and get something working fast. Such an exploratory process can lead to overly-complex software. I guess the trick is to have the discipline to go back and do the "science" to distil and refactor.