Re: SDLC?
Yes, and then there's bespoke application code, requiring updates to business logic to keep in line with current legislation, written in languages that only those whose IT career precedes Java can support and how much it costs to tempt them out of retirement, or compiled on processor families that are no longer manufactured, with a file format that only a previous generation of tape drive can read and a user base that would take months of retraining per person to migrate off of it.
You know that quote about when you owe the bank £500 you have a problem, when you owe the bank £500 million, the bank has a problem? Extend that to technical debt. Past a certain point it's not even an IT problem anymore.