Actually, the source code for most things that aren't implemented in C are available. They're just distributed in the compiled MX form, so there isn't a source file to read. But you can still easily 'disassemble' parts of the language at runtime.

Here's a very simple way to do it for DateListPlot:

   Get["https://raw.githubusercontent.com/szhorvat/Spelunking/master/Spelunking.m"]
   DateListPlot;
   Spelunk[DateListPlot]

This produces a hyperlinked 'code browser' in your notebook that lets you chase down function definitions out of the live kernel. Here's an example of what it looks like:

http://imgur.com/ZjWJvEO

So the OC could actually rewrite DateListPlot to add error bar support, if he wanted to. He could even overwrite the original MX file in his installation once he had done this, if he so desired.