>You don't, of course, but it's usually a pretty reasonable assumption that the effort involved in implementing your own (reliable stream sockets|scheduler and memory manager|high level language compiler) will eclipse severalfold any advantage from doing so.

Wrong. I implement network protocols for a living, and I use often use UDP because TCP is not sufficient for the purpose. So yeah, this is the problem: your conceit that your upfront design is going to be "enough" for everything.