Joel has since said that that he doesn't really agree with that advice anymore, at least not in the same way. Super annoying that it gets parroted over and over again as though it's the word of the lord.
The balance is somewhere in the middle, it's valuable advice for what can go wrong when you don't think through the implications of the decision making process, understand how and why the system works the way it does, and what risks exist for what can go wrong with a big redesign. But like anything, if the risks are understood, then those risks can be accepted, mitigated, or rejected if appropriate, or provide guidance on why the redesign investment isn't worth the associated risks.