From David Sedley, Plato's Cratylus:
There have been some recent proposals to jettison this whole chronological structure [of the development of Plato's thought], but in my view they represent, so far at least, little more than the understandable fact that people are getting bored with it. There is actually much to lose if we say goodbye to it. For by reading Plato's development along the lines I have summarised, we are enabled to understand how the youthful admirer of the maverick critic Socrates became in time the teacher of Aristotle and the august founder of a metaphysical system which was to dominate philosophy for the last half-millennium of antiquity and well beyond. (7)
The problem with this argument is that it mistakes the assumptions underwriting this kind of chronology for the conclusions it provides. The "development" theory of Plato's philosophy assumes that Plato developed from an admirer of Socrates into a full-blown metaphysician who bequeathed to Aristotle, among others, explanations and problems Socrates did not conceive of. In order for the development theory to be plausible, it has to assume a development.
Now when Sedley states that "by reading Plato's development along the lines I have summarised, we are enabled to understand how the youthful admirer of Socrates . . ." he is mistaking the conclusion of the development theory for its assumption. The only way the development theory "enable[s]" us to understand Plato's development is by assuming that Plato's thought developed.
But what if Plato's thought did not develop? What if, per Schleiermacher, Shorey, Kahn et al., Plato's philosophy stayed pretty much the same from the beginning of his philosophical career to the end? Call this the "stasis" theory. Now suppose I said something like what Sedley says, only in favor of the stasis theory: "By reading Plato's largely unchanging philosophical outlook in the ways suggested by Schleiermacher et al., we are enabled to understand how Plato arranged his dialogues in such a way that it only appears to the reader of his corpus that his thinking changed."
I do not know which of the basic hypotheses about Plato's development (or lack thereof) is correct, but it seems clear that the development hypothesis has become so dominant in Plato scholarship that its assumptions are being mistaken for its conclusions.