I've dealt with some very competent people, and seen many incompetent ones. I'll readily concede the point.My guess is you havn't dealt with many C++ programmers. Trust me, not many are competent.
You make a good argument, and I don't completely disagree with it, I just think it is too strong of a statement. My assumption is not that you have hired a set of code monkeys, but rather a fairly competent set of computer science types. Part of the time will be making sure that the developers are proficient enough in the language to work with it and the available tools (if available) to deal with the language; part of the time will be spent finding the neccessary libraries and learning them, assuming they exist. For a large and complex enough application, there may be enough separate modules that could each be done in an optimal language that hiring or training enough developers in the specific languages may be too expensive, too time consuming, or those folks may be too rare.
There is an additional factor as well. Say there are many competent developers in C/C++/Java/LISP/Ada/etc available, but few in the 'optimal' language. You're company may not be in a position to hire enough people who know that language at the time, and may have to settle (perhaps only temporarily) for a slightly less than optimal solution.
So, I'm not saying that C++ and its kin are the cure-all. They certainly are not; and there is certainly a lot of code written in those languages for the reasons you mentioned (primarily ignorance). In your example of Erlang, if your company is going to be doing primarily telecomm work, then it'd be foolish to use C++. However, if all you'll ever need is a small telecomm module, and competent C++ programmer may be able to design and develop such a module in C++ that is extensible, maintainable, etc in less time than he'd take to learn the new language.
As for the matter of the open source team, I completely agree. If you're just doing it because you want to show it to the world and you have (essentially) unlimited time, then yes, do it right.
In summary, I find this too strong of a statement, but not completely unjustified. There are more factors to consider.There are a few situations in which usign the less optimal language is acceptable. The most immediate one that comes to mind is client specifications. This comes out of the ignorance statment of my previous post. Some clients want their product in a certain language and won't be told no by anyone.
By and large, yes, they do seem that way.I hope those arguments seem fair and justified.