If server-side (web)apps were written in C++ instead of Ruby/Python/..., I believe they can sustain much more users before they need to consider horizontal scalability seriously, since we can fully utilize the machine in C++. AFAICT it is nearly impossible to optimize to the level of cache efficiency (and etc.) in Ruby/Python/..., and a distributed system is much harder to cope with, and when you need to work on that, the choice of programming language becomes less important.

However, building a not-so-bad prototype in Ruby/Python/... is much easier. So probably something needs to be done (e.g. "frameworks", libraries, tutorials) for C++.

[EDIT] C++ (templates) can provide abstractions with zero run-time cost. C doesn't have such abstractions. Ruby/Python/... don't have such low cost.