What do you mean by "validity"? If you're asking whether there are any modern historians who believe that history progresses cyclically, well, you'd struggle to find anyone who does. But that doesn't mean that Polybius' ideas were invalid or wrong. He interpreted history using ideas which were common during his lifetime. We might not agree with his interpretation, but people in a few hundred years time will probably disagree with many of the ways history is interpreted today. It doesn't make our ideas invalid.

Greek history and political thought tended to be highly schematic, looking for generalisations and patterns in order to attempt to predict the future. If you haven't already, I'd suggest reading the passage of Polybius in question (6.3-9), because he does demonstrate his thought process quite nicely. His ideas seem to be based partly on his knowledge of the history of various Greek states, although he doesn't go into much detail on the particulars (6.3), as well as earlier philosophers (Plato is mentioned). There's also an element of rational "common sense": he traces the course of a hypothetical state, explaining how each stage comes into being and then becomes corrupts, seemingly based on the idea that power corrupts (6.5-9). The whole purpose of his argument is to show how unstable most states are in contrast to the Roman Republic, which he sees as a perfect balance of the three "good" forms of politeia or constitution (kingship, aristocracy, democracy). The consuls, senate, and people respectively balance each other out, preventing any of them from dominating, and thus the Roman state does not progress through the cycle, and retains the internal stability required to conquer most of the Mediterranean (6.11-8). Clearly Polybius doesn't think the cyclical theory applies to every state, as the Roman Republic is excluded as being a mixed constitution (and so is Sparta, actually). The cycle of constitutions is what a modern historian would consider a model: it doesn't apply in every scenario, nor does it perfectly represent every aspect of reality, but it may be a helpful way of looking at things.