Many probably noticed that very old, relatively simple machines, cars, tractors, printing presses, etc. can be maintained almost indefinitely. Many businesses do prefer old machines even when there are superior new alternatives and that is because the new versions are just too complex.
To keep maintaining something indefinitely you need to understand the machine enough to know every single piece that needs to be maintained and how to do it properly. Everywhere you need to lube it or clean it or to look from time to time.
Make something complex enough and nobody will have this knowledge. Things start deteriorating. Maybe the data in the database will start accumulating defects, maybe those defects start breaking processes, maybe the processes will not be fixed properly but the errors just get ignored possibly causing other data inconsistencies, and so on.
When you have people who wrote it and understand it fully they can very likely maintain it properly because they understand how to work with it, where to look, how to address problems. Loose them and if the system is complex enough the new crew may not be able to acquire this ability. And then it is just the question of how fast the system is going to be deteriorating.
Mostly I’m thinking of more generic services rather than pure ‘business logic’ systems. These are the sorts of problems that Twitter might have in the future. Some of these are problems that Facebook might have gotten to discover in their last big outage.