Kristen Maevyn's essay "The Decoherence Trap" names the cost a system accrues when it holds several valid but mutually incompatible models, so every change becomes a job of translating between them and guarding against side effects.
In "The Geometric Model of Maintenance," someone finally checked the “one month of code, ten days of maintenance” rule against real git history. The number holds up. What it’s made of is worth a look, and so is what you’d need to measure before it tells you anything about your own code.
A Reddit user on r/SaaS is looking forward to the consulting rates he'll command once the wave of projects trying to dig out from under AI slop hits. He's not wrong, but he's dated the pattern to 2010 offshoring when it's older than that, and the lesson never sticks: coding was never typing.