Indeed. If i cannot understand a system (in my field) in a couple of days (depending on the size), and have been asked to work on it, chance is i'll ask for a rewrite, with documentation and readable code.
If your system is not readable and undocumented, don't blame the guy who will come and decide to rewrite it.
So you have a legacy system that is being used by other systems, you're the new super genius and can't understand the system, so you rewrite? Discounting the potential release iterations and bug fixes that are in the legacy system? Your "improved" version can't include that knowledge since you didn't understand the legacy system. Also discounting the company wide knowledge of the legacy system, warts and all that exists? Also having two identical systems isn't fantastic. Rewriting a system because someone can't understand it is rarely the correct solution. It most likely means the rewriter doesn't understand the problems that are being solved by the legacy system either.
And chance is, when you meet the 62k rows of code authentication system that is way too complicated and does way too much, that you will not be permitted to rewrite it in a more sane and modular way.
He didn't say this system was unreadable or undocumented, only "complicated and creaky," One particular programmer who was regarded as "smart" couldn't figure out what it did, but it may just have been too complex for him.
If your system is not readable and undocumented, don't blame the guy who will come and decide to rewrite it.