I will also just add two more notes, since you raised the issue of keeping old users:
1. Old users are used to not having syntax highlighting, I don't see how not adding it would make them leave. The larger barrier, as Riamus said, is the lack of other clients. If a veteran user left because mIRC lacks syntax highlighting, where would they go? I can't think of any client that competes, on Windows, and most others don't even have built-in editors in the first place (irssi, etc.).
2. The older users will know about the plugins and scripts that can add highlighting. They've been around the block, presumably they've done their research. Your point about not knowing about these scripts really only applies to newer users, which brings me back to: new users aren't picking up mIRC for the syntax highlighting. It might make it easier for them to learn, but this wouldn't be a stay or leave scenario.
I'm also for adding syntax highlighting to mIRC (that's why I wrote the dll), but from my experience implementing it, I don't think it's worth the effort for the minimal value add.
Also, I just saw this: "even QBasic back then had syntax highlighting". That made me double take. I don't know where you got that information, but QBasic never had syntax highlighting. This is the editor everyone saw:
