That's true, but I'd guess that's true of much desktop software. People don't use Microsoft Word because of its efficient C++ code; they use it because it's semi-standard, has lots of features, and overall is good enough. I would bet giant piles of legacy code are a bigger reason for not moving to C# than anything language-specific is.
Don't look to Minecraft for an example of well written code, there are open source alternatives (Minetest in C++ springs to mind) that run rings around it. And Notch himself is well known for his inefficient magic-number and circular-reference ridden Java code. Although I can't attest to Jeb (who is now the lead dev)'s coding skill.
