At least thrice I joined teams where all original developers and the manager was long gone. The situation for some reason looked very similar in all cases. The team is complaining about supposedly bad design that is preventing them from getting work done. But the system is also large and complex and they are unable to fix/rewrite it.
Lo and behold, in two out of three cases I found that the original developers actually did pretty good job designing the system but in all cases any one of new developers understands only small part of the design and is unable to contribute changes across the entire system, there is a core of the system that nobody understands even though everybody uses and the team has made changes that are against original ideas of the system quickly creating more problems and exploding complexity.
In all cases developers are only able to comfortably do some kinds of changes to the system that are routinely done (like adding new fields to forms, messages, etc.) that do not change how the system functions. Changing the basic structure of the system is a taboo.
In essence, developers treat "legacy" code as if it was some kind of framework or language runtime -- to be used but never to be studied to have its internals understood. (I will never understand this approach to development)
Making "non-standard" changes (yes, one team used this exact term!) pretty much always ends up with something resembling long archeological expedition with ancient traps, diseases and wild animals.