The longevity of code depends at least on whether it's a product or a service.

Services are what the majority of devs already work on and maintain. There's almost no incentive for anyone to use LLMs for that outside of startups. They do indeed last a long time because the code is as fundamental to the recurring revenue of the business as their legal or accounting or marketing. Devs make changes according to the evolving needs of the business, and "productivity" isn't as much of a priority as accuracy and reliability. The implementation details are very relevant to the business, especially for B2B services that need to meet compliance requirements.

Products, however, have always been disposable code written by people being thrown into a meat grinder. I don't think LLM-generated code is better, but it's probably not that much worse either.