The chance of a defect fix introducing a new defect tends to grow linearly with the size of the codebase, since defects are usually caused by the interaction between code and there's now more code to interact with.

If you plot this out, you'll notice that it eventually reaches > 100% and the total number of defects will eventually grow exponentially, as each bugfix eventually introduces more bugs than it fixes. Which is what I've actually observed in 25 years in the software industry. The speed at which new bugs are introduced faster than bugfixes varies by organization and the skill of your software architects - good engineers know how to keep coupling down and limit the space of existing code that a new fix could possibly break. I've seen some startups where they reach this asymptote before bringing the product to market though (needless to say, they failed), and it's pretty common for computer games to become steaming piles of shit close to launch, and I've even seen some Google systems killed and rewritten because it became impossible to make forward progress on them. I call this technical bankruptcy, the end result of technical debt.