As a guy in his mid-forties, I sympathize with that sentiment.

I do think you're missing how this will likely go down in practice, though. Those giant codebases with years of domain rules are all legacy now. The question is how quickly a new AI codebase could catch up to that code base and overtake it, with all the AI-compatibility best practices baked in. Once that happens, there is no value in that legacy code.

Any prognostication is a fool's errand, but I wouldn't go long on those giant codebases.