At a previous employer, I left behind maybe not millions of lines of Python 2 code, but certainly somewhere in the hundreds of thousands range. There was also a fair amount of reliance on 3rd party libs for databases, IBM MQ and tibrv that had native components. We also had our own Python libs that had native components written using Boost Python.

Large Python projects are possible, but if you don't have good discipline and organization, it can quickly become a mess (probably true regardless of the language).

I'm not sure how much effort it'd be to migrate all of that to Python 3, but I'm sure it's not trivial. But, I also don't think it'd be a multi year effort. It also wasn't a big monolithic project, so a migration could be done piecemeal.