So? What came before capitalism? Feudalism. That was a class society at least as much as capitalism is.
BTW: Why "contradiction"? It looks like Marx chose that word in order to make the fall of capitalism look inevitable - it has contradictions, so it must be self-inconsistent, so it must fall, right? But it looks to me that what Marx is describing is just the differences that occur in any society. In what way are they truly contradictions (in the normal sense of the word)?
Contradictions doesn't mean internally inconsistent in dialectical sense; Marxism is famous for its application of dialectical logic, in which opposites oppose each other while maintaining unity which must resolve itself, rather than a binary opposition (i.e we must have one or the other) or a binary inclusive-or (which has no potentiality for resolution).
The point of contradictions isn't to say that society will fall because of them (that would be a vulgar reading of the concept and a scientifically untenable one) but rather that any new society must in some way answer to the contradictions, or arise out of their development or increase in intensity. For instance, the contradiction between use-value and exchange-value is only heightened with capitalism, but the contradiction between capital and labour is obviously new with capitalism. By specifying contradictions, they can be thought of (philosophically speaking) and potentially overcome (practically speaking). One such contradiction which has been present in all societies thus far is between freedom and necessity, which Marx claimed would be resolved with the advent of Communism.
I can't speak for the impression Marx wanted to give about the downfall of capitalism from his word choice, but I can say that the dialectical mode of presentation was admitted by Engels to have been rather outmoded even by the time Capital I was published. Contradiction in the dialectical sense doesn't imply inconsistency, it implies the functioning unity of opposites. At best, Marx's use of contradiction implies the possibility to overcome, but I don't know about the necessity. Herbert Marcuse, a renown dialectician, didn't seem to think that the contradiction would be resolved, and Adorno didn't seem to think that the contradiction would resolve positively.
Either way, I'm not going on the defensive here, since Marx's commitment to the scientific method should be reflected in Marxism's ability to be self-critical. Few other "sciences" would be so happy to cast doubt on the logical axioms under which they operate.
