Complexity itself isn't evidence for design. I can think of many examples of complexity that are neither supernatural nor purposeful, starting with the chance oilslick on a rain puddle. In the end, all Behe can do is suggest several experiments of dubious premise. For example, he wonders if some primitive ancestor had all the instructions for today's diverse, multitudinous organisms somehow prepackaged, waiting to go. What we already know about organisms, information systems and genetics makes that idea patently silly.
Purposeful design is poor science because ultimately it's religion.
Behe may be a fresh actor in a new production, but it's an old play. Behe can call his designer whatever he wants he's still resorting to a supernatural explanation of the natural world. Science explains the natural world in material, naturalistic ways using observation and measurement. Because the supernatural is untestable, it's automatically out of bounds. Confronted with this barrier, creationists are trying to change the rules.