Statistically speaking, most codebases are brownfield. Without joining a startup, working on a greenfield project is actually a pretty rare treat. And deleting code is wonderful. I'd go as far as to say lines removed is more often than not more valuable than lines added; every line that's no longer in the codebase is code you no longer have to understand or care about. So long as functionality remains intact, of course.

The problem, as I see it, with prolific use of AI to generate code is that it goes in the exact opposite direction. More and more code is bolted on top of existing code, more and more edge-cases, patch-ups, workarounds, etc. accumulate, the codebase grows and grows. In the end, no matter how good you are at understanding "code from questionable sources", you're still a human being. The AI can generate new code at rates several orders of magnitude faster than you can injest and understand it, and when your meat brain becomes exhausted, the machine does not tire. From a business perspective, your employer will weigh their options: they can wait for you to interpret the code and generate good code (whether by hand or by machine + human review).. Or they can just keep pulling the lever on the slot machine until it works well enough to sell. And for the business exec just looking for the fastest path to paydirt, I'm afraid the latter option is going to look way more appealing.