Why would Apple Not tweak the A4's instruction set?

Let's take an open source software analogy. I got this wonderful code from a group that is really picky about patches but has a good plug-in architecture.

I can change the core code. When new software shows up, I have to try and re-patch the software to add back in my core changes. I might try to submit my changes to the group, but if they are not broadly useful, they won't get adopted.

I can add my stuff through the plug-in architecture. It is a known expansion path, cheaper, and has less of a chance of breakage due to it being the preferred, documented method. Sure, when new software shows up, I will probably need to make some changes, but it will be easier.

However, extending the processor itself goes against the entire RISC philosophy. Making the core more complicated makes speeding it more difficult and will not produce significant gain unless the additional instructions are very common.

A more complicated and less orthogonal instruction set also makes the compiler's job much more difficult.