This seems like your mental model assumes relatively small codebases. On a codebase with 1m LOC, your estimation of 2500 lines&#x2F;day still takes ~400 person-days. That&#x27;s a big investment to put into something that doesn&#x27;t present clear benefits. Moreover, converting a huge codebase piecemeal is something of a challenge; I don&#x27;t know if it&#x27;s even possible to have python 2 importing python 3 (or vice versa) but assuming your large codebase is relatively interdependent, reorganizing the codebase such that you can split it into modules which can be migrated independently could also be a big undertaking. I would also anticipate that there would be a long tail of fixing bugs caused by incorrect migration (are we code reviewing those 2500 lines&#x2F;day?) as well as lower productivity as developers get used to the new language.<p>This is why you frequently hear that large, established codebases that control their environment aren&#x27;t updating to Python 3. It&#x27;s pretty easy to move a few thousand lines over but complexity scales non-linearly with the size of the codebase.