Forgive me for cliche scientism, but I recently realized that I can't think of any major philosophical developments in the last two centuries that occurred within academic philosophy. If I were to try to list major philosophical achievements since 1819, these would likely appear on my list, but none of them were from those trained in philosophy:
A convincing, simple explanation for the apparent design we find in the living world (Darwin and Wallace).
The unification of time and space into one fabric (Einstein)
A solid foundation for axiomatic mathematics (Zermelo and Fraenkel).
A model of computation, and a plausible framework for explaining mental activity (Turing and Church).
By contrast, if we go back to previous centuries, I don't have much of an issue citing philosophical achievements from philosophers:
The identification of the pain-pleasure axis as the primary source of value (Bentham).
Advanced notions of causality, reductionism, scientific skepticism (Hume)
Extension of moral sympathies to those in the animal kingdom (too many philosophers to name)
A highlight of the value of wisdom and learned debate (Socrates, and others)
Of course, this is probably caused my by bias towards Lesswrong-adjacent philosophy. If I had to pick philosophers who have made major contributions, these people would be on my shortlist:
John Stuart Mill, Karl Marx, Thomas Nagel, Derek Parfit, Bertrand Russell, Arthur Schopenhauer.
* The development of modern formal logic (predicate logic, modal logic, the equivalence of higher-order logics and set-theory, etc.), which is of course deeply related to Zermelo, Fraenkel, Turing and Church, but which involved philosophers like Quine, Putnam, Russell, Kripke, Lewis and others.
* The model of scientific progress as proceeding via pre-paradigmatic, paradigmatic, and revolutionary stages (from Kuhn, who wrote as a philosopher, though trained as a physicist)
8TAG
Wow! You left out the whole of analytical philosophy!