I agree with this statement in some ways, but disagree with it in others.
First, I definitely agree that Marx’s failure to see that capitalism could evolve makes it much less likely that his theory will ever come to fruition. Marx believed that the contradictions inherent in capitalism would cause it to fail. He also believed that the proletariat would eventually rise up and throw off the capitalist system. He regarded these developments as historical inevitabilities. Now, more than 150 years after The Communist Manifesto, capitalism is very different than it once was. The changes in capitalism make it much harder to imagine the workers rising up and overthrowing the owners of the means of production. The changes in capitalism have made it much more humane and less likely to instigate revolution against it. This is likely why the only successful communist revolutions have been in the least developed countries.
Second, I disagree with the idea that this means that Marx’s theory is now completely irrelevant. The theory will be relevant for quite some time because it has had a real impact on the world. For example, China’s government, though it has gone away from Marxism for the most part, is still influenced by Marx’s ideas. We cannot understand our history, or even some aspects of our present, without understanding Marx.
Perhaps more importantly, Marx’s theory will remain important as a critique of capitalism. Although capitalism seems to be succeeding as an economic system, it is clearly a system that has many problems. Marx helps to point out what some of these problems are. This will continue to be important because it can spur us to try to improve capitalism and continue to make it less abusive and exploitative.
So, Marx’s theory is now irrelevant in that it will not (in my view) ever come true in the way he predicted. Even so, it is still important for the way it has impacted our history and for the fact that it leads us to think critically about capitalism and capitalist society.