This seems like your mental model assumes relatively small codebases. On a codebase with 1m LOC, your estimation of 2500 lines/day still takes ~400 person-days. That's a big investment to put into something that doesn't present clear benefits. Moreover, converting a huge codebase piecemeal is something of a challenge; I don't know if it'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/day?) as well as lower productivity as developers get used to the new language.

This is why you frequently hear that large, established codebases that control their environment aren't updating to Python 3. It's pretty easy to move a few thousand lines over but complexity scales non-linearly with the size of the codebase.