I've never worked for a company of that scale, so I don't know how hard it'll be to find legacy developers, but I imagine it'll take at least several years to port a public-facing system to Python 3, if they were to start now.

If they start in the next few years, I imagine it'll be much harder to find ready and willing Python 2 developers with the experience to navigate such a big code-base? They could hire consultants, but I imagine the future hiring cost will also be astronomical.