Technical drift
Your application rests on dozens of software building blocks written by other people. They keep moving without you. Every month of waiting widens the gap, and past a certain point the update is no longer an update: it is a rework project, priced as such.
Security holes
When a flaw is found in one of those blocks, it is published. That is useful information for whoever updates, and a set of instructions for everyone else. The risk therefore does not come from your code, but from what you have not updated.
AI models going obsolete
If your application calls an artificial intelligence model, it depends on a service that changes version, revises its prices, or drops a model from its catalogue. A feature that worked yesterday can stop overnight without anything changing on your side.
Jobs that stop silently
An automatic send, an overnight sync, an invoice produced every month: when one of those falls over, nobody gets an error message. It surfaces weeks later, while someone works out why the figures do not add up.