"crazy old" likely has years and years of people documenting all the workarounds and hacks that need to be done to achieve X
while "crazy new" might not have that much cruft, but you'll be stuck trying to figure it out by pretty much alone
but of course it also depends on how clever the people beforehand have been, is it stuff tied to impossible knots that cross over 5 parallel dimensions, where nobody knows what it does or how it's doing it, just that "it somehow works, so we don't touch it, that guy was a wizard", or is it just layers of faith held together with duct tape and rope, where people tried to fix stuff over years, throwing in patches that "probably should help, I think, maybe", which could be condensed into less than a third of the size, when rewritten...
but of course it also depends on how clever the people beforehand have been, is it stuff tied to impossible knots that cross over 5 parallel dimensions, where nobody knows what it does or how it's doing it, just that "it somehow works, so we don't touch it, that guy was a wizard", or is it just layers of faith held together with duct tape and rope, where people tried to fix stuff over years, throwing in patches that "probably should help, I think, maybe", which could be condensed into less than a third of the size, when rewritten...