Kennedy is the man. There's not much else to say. Sure, the thesis is "simplistic" or "schematic" or "deterministic" or whatever other bombastic term pseudo-intellectuals (and legitimate intellectuals alike) choose to use, but one has to understand that the project of this book is to make some sort of reasonably defensible generalization about what leads to the fall of great powers throughout history. That's not exactly easy. No single explanation will be perfect. But his thesis about imperial overstretch is instructive and a useful lens through which to view the powers over the years.
His predictions, as we have seen, have proven far from true, but, hey, he's a historian, not a clairvoyant. If masterful analysis of the past meant accurate prediction of the future, things would be very different.
The real strength of this book is not even in its overarching argument but in the sheer synthesis and presentation of so much useful data about great powers from 1500-2000. It's not easy to find a one-stop book for economic and military data on 16th-century imperial Portugal and 1950s USSR, but this book has it. It made his career, and he surely deserved it.