Why wait 15 years? If it's becoming too big of a ball of mud, tell the AI that, and have it ultrathink a plan on what to refactor together and better follow DRY principles of clean code, or whatever you adhere to. Tell it hey, I noticed that we're reimplementing the same thing a couple of times, let's make it a class that can be reused instead. It'll happily generate cruft. The human in the loop should call that out so it doesn't get out of control. Maybe that's what we'll be paying the human to do still in 15 years.