I think in the above sentence might lie the clue to what you are missing about Russell (and why)!

Not much would be gained in any case by rephrasing a question about the greatness of a philosopher in terms of the equally slippery quality of irreplaceability, but Wittgenstein is an especially inappropriate subject for this jeu d’esprit, because as soon as you actually look around for a possible replacement, you find Frank Ramsay, Wittgenstein’s thesis adviser, friend, and translator, and a genius himself, finely tuned to the frequency of Wittgenstein from the moment when he discovered the Tractatus and translated it for the first English edition, at age 19. Anyone who understands Frank Ramsay’s profound contribution to logic, economics, and probability, all accomplished before his death at 27, will find it easy to believe that he might have continued Wittgenstein’s work in something very close to the same spirit, if Wittgenstein had perished in 1930, and Frank Ramsay had survived.

And, you can do this zooming in on so many issues. It’s almost surprising when you find something sketched out in the “big picture” that you don’t find original and illuminating discussion somewhere in Lewis (one example: I don’t think he really ever did more than sketch how the reduction of the intentionality of belief content was supposed to go, in contrast to the details given for the language case).

And you can zoom past the main thrust of the papers to find almost throwaway remarks and insights that in others work would count as whole papers (I’m thinking e.g. the sort of stuff in the appendices to general semantics on degree-truth for vague predicates). And this is without taking into account papers that are self-standing gems outside the “big story”.

I just can’t think of any other philosopher that’s so thoroughly good at each level of resolution.

As an example of the former, take the idea that understanding a sentence consists in knowledge of its truth-conditions. In his survey of 20th century phil-language, Jason Stanley describes this idea as ‘extraordinarily fruitful, perhaps the most fruitful insight in the long history of the study of meaning’. Of course, it’s an insight that goes back further than Wittgenstein. But we do find one of the earliest and clearest developments of this idea in the Tractatus. Jason points out that the Tractatus also made connections between modality and issues of content that transpired to bear fruit when systematically explored by later philosophers.

There’s also some timely stuff in the book about the demarcation of mathematics from logic, most of it a reaction to Russell helping himself to what we now naturally think of as set-theoretic resources to carry out the logicist program. (These issues haven’t gone away, btw. Think of the ongoing debates about the status of second-order logic.)

I think there’s also room for disagreement about the extent to which the other parts of the book lead philosophy in the wrong direction. In the paper mentioned above, Stanley takes the book to overall have had a negative impact on 20th century philosophy, since it sparked a hunt for a criterion of meaningfulness, and we all know how well that went. I’m sure some would have similar criticisms of the book on the grounds that to some extent it ushered in the linguistic turn (the seeds were sown in Frege’s work and elsewhere of course). To the extent that one things these were horrible wrong-turns in 20th century philosophy, one is likely to think that their Tractarian roots were rotten. This isn’t an attitude I share, but it seems fairly widespread.

I won’t even attempt the later Wittgenstein. I tend to incline to the view that a lot of the issues brought into focus in the Investigations and elsewhere get discussed a lot without people being willing to recognize their roots. For instance, contemporary philosophy of language largely marginalizes ignores the rule-following considerations. (Notice, for instance how large they loom in Hale and Wright’s 1997 Blackwell Companion compared to the 2007 Lepore and McLaughlin Oxford Handbook). But I think a lot of the issues the rule-following considerations raised, for instance those about tacit knowledge and how it can play a role, are getting discussed at present in the debate on knowledge how. (I note also that epistemology inspired by ‘On Certainty’ is thriving).

Just to anger some Wittgensteinians into responding, let me say that the Tractatus seems to me sort of a dead end. If I’ve ever argued with anyone who used the early Wittgenstein to advance their case on a contemporary issue, I was oblivious to it. And while the later Wittgenstein has plenty of big fans, I sometimes wonder if the allure of his style makes people fly the Wittgensteinian flag over ideas that they didn’t really need him to get to.

Aside from this, the Principia influenced the way we do mathematics to an extent that is comparable to Russell’s influence on philosophy itself. Granted, he drew on the works of Dedekind, Cantor, Frege, Weierstrass, Peano, and others, but this is how mathematics works (and philosophy too). How many people are able to stand on the shoulders of giants and see further, after all? And of those who can do it, how many see as far as Russell?

Another thing which contemporary philosophers seldom remember is that Russell refuted the idealist arguments against the logical consistency of space and time. This may sound like small stuff, but in fact it was a huge issue, and the only reason why we have a hard time noticing it is that is has become a part of the philosophical air we breath — we now “simply know” that space is logically coherent.

Another thing that is very often neglected is Russell’s later work, which traces his transition from a Cartesian to a third-person epistemology , and from his well-known internalism to his widely neglected causal conception of knowledge. 75 years old, he was still decades ahead of his contemporaries, doing what was basically contemporary M&E while everybody else was doing one kind of linguistic philosophy or another.

(While we’re at it, don’t forget that, for better or worse, Russell was at the root of both logical empiricism and ordinary language philosophy — the latter of course together with Moore.)

For sure some form of analytic philosophy would have emerged from Vienna without Russell and Cambridge in the picture, but the resulting shape of Anglo-American philosophy is very very different in that possible world.

By the way, what is the case for Wittgenstein exactly?