When will support stop for Python 2.7? Once it stops being patched/supported, it represents a security risk (in my book).
Additionally, eventually new libraries will also stop supporting 2.7, with the same issues as well as making development harder (or not as easy as your peers/competitors)
The Python development team will cease supporting Python 2.7 in 2020. After that you will have to buy support from someone like Red Hat or Continuum Analytics.
Open source support is not a centrally governed entity. FOSS support for Python 2.7 will continue after 2020. You will certainly still have the option of paying someone for support.
Sure, there's a chance people will pick up the mantle and essentially fork Python 2.7 to do unofficial bugfix releases past 2020; as you pointed out, it's open source and people can do what they want. But my point is there is no guarantee that will happen, nor that those unofficial releases will be of quality either. RH and Continuum on the other hand will most likely have financial incentives to make sure any maintenance they do meets the needs of their paying customers and thus are of a certain level of quality. IOW one possible future is more of a certain than the other.
Additionally, eventually new libraries will also stop supporting 2.7, with the same issues as well as making development harder (or not as easy as your peers/competitors)