Yes, the middle ground has some disadvantages of both extremes, but it has some of the advantages to both.
C++ maps to hardware well. An enterprise web solution, doesn't need that. It should have automatic memory management. The scripting languages are not strongly typed. That makes the code a little less maintainable and a little slower.
When Twitter used RoR, it was a nightmare. It just wasn't performant enough. When they went to Scala things got a lot better.
Would Twitter ever want to use C++? I doubt it. They are probably a bit hardware agnostic and they don't want to deal with the classes of bugs that C++ has.