I suspect we are approaching a world where this makes sense to do annually. Opus 5.5 is good enough to improve on most codebases, and Opus 7 will no doubt be good enough to improve on that.

Current models are amazing but production code incorporates unknowable information through decades of diagnosis and bug fixing (and sometimes the opposite: working around bugs in other software that it talks to). Put even the most amazing coder that was ever born onto improving a complex old codebase, and you can guarantee they are going to break stuff. AI is better but not infinitely better.