It seems like you're assuming that the only way for it to get better is more human-written code. Also, why would software quality get worse over time, as opposed to stagnating at worst? Attempting to use it for more complex applications?
There is already so far beyond enough data available online for an AI to be a super-human senior software engineer/architect/whatever. Because there's enough for the humans, and an AI can do it better in theory. It just needs to be a good enough model.
Barring the most extreme edge cases, it's never going to be worse than it is right now.