I think, your idea of "modern" is not very practical, when it comes to computational mathematics. To people, who do research on algorithms, the one crucial measurement of their success is running time. This is also very important to most mathematicians, because hypotheses are tested for very large numbers/objects. Your "modern" languages are mainly geared at speeding up the design and coding process and not the running time of the programs. Implementing mathematical algorithms in C# would have more of an educational purpose than a practical one, and if someone tries to sell you such a thing, you shouldn't buy it.
I don't know much about the "common knowledge" that you are referring to, or in fact about the software industry, but fact is that when people write computer algebra packages, they still sometimes implement the crucial and most used parts in Assembler! Your comparison "Java versus C++" misses the point. Already C++ produces so much more overhead than C, say, that it would rarely be the language of choice for computer algebra packages, where efficiency is crucial. Don't get me wrong, I recognise the huge advantages of object oriented programming languages, but they are at their best if they are used for the things they were design for.