The problem is that most code is utter shit. If you just read shit code and do nothing, you have made a choice to preserve the status quo. At that point, you have become part of the problem.
If you do a from-scratch rewrite, you could stay in igorance and might make those mistakes. But if you do a refactor, especially one broken up into many steps like this Linux one, it in fact demonstrates a better understanding the existing code and any nuance than merely leaving it in place!
Not touching scary-looking code is just bad, and not a wise form of humility.