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.

Well Python 3 came out in 2008, this date is known since 2014. If they start in the next few years, yes you'd expect it to be expensive.

But distribution vendors like Red Hat and Ubuntu have Python 2 in long term support distributions that they will give support for a few more years, so it is possible to use those. And after that they offer more years of support to paying customers. It's not necessary to hire programmers to do this for the companies needing support themselves.