Lots of developers (especially newer ones) seem to think code is "utter shit" when it really is written that way for a reason. At least I've seen this pattern many times, and used to do it myself decades ago.
Despite the implication otherwise, I have been a software engineer for 30 years. This is speaking as an active consultant in the embedded space. There is shit code everywhere, in nearly every single system. Some code may be "written that way for a reason", but it can simultaneously be utter shit. Working shit, but still shit.
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.