Well if you're looking for anecdotal evidence: my last hobby project (OCR for a specific book+script), I used Mathematica to prototype (fast iteration) and C++ with OpenCV to implement it (way faster execution).
I started the project in Python but switched because it wasn't particularly good at anything vs. the combination of Mathematica and C++. Actually I prototyped it in C++ too before doing some extra analysis in Mathematica only because it had some algorithms that OpenCV didn't.
I'm interesting in your background: which languages do you use, and what kind of programs do you write? Perhaps it'll become clearer whether C++ is any use to you or not.
From the discussion I've read in this thread so far, it seems like you're claiming "what's the point of the entire field of mechanical engineering" when you, personally, just want to drive a car (not build one), fly on a plane (not build one) and so on.
What I would love to see is something that replaces Chromium with WebKit/Safari in Electron apps. I'm sure that'll make everything faster, or at least easy on battery life.
It might take only 30 sec to tweet about it, but with open-sourcing comes a lot of stuff to deal with: once it goes open, there will be forks and pull requests and what not. Apple probably isn't yet ready to do fully open development (like they do with LLVM) of Swift.
Give it time, I think it will be open. It's just not 100% finished yet, despite what Apple says.
The Xcode 5.1 sources include clang, lldb etc. We just need to wait for Xcode 6 sources to be up, pretty sure it'll include Swift.
Also worthy of note is that most of these packages are under BSD/APSL license, which doesn't mandate Apple to release the sources at all (e.g. the OS X kernel xnu is fully open source). Yet they do it. Google's version of "open" Android development isn't any different, they develop in private and dump the code to AOSP only once a particular version is 'finished'.