Well, the largest companies in the world don't have to rely on volunteers, they can pay for continued support.
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.
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.