Cellline contamination is a real problem where a foreign population out competes the intended cell line. There are estimates that thousands of papers have been published where the intended cellline has been accidentally replaced with HeLa.
https://en.wikipedia.org/wiki/Canine_transmissible_venereal_...
This is not true. Most cow cells will almost immediately die, unless they are attached to a cow. Even if plenty of nutrients are available.
This is a basic anti-cancer mechanism, cells need to receive specific signals from nearby cells to stay alive. The default state for a cell is "dead".
There are ways to work around that, and the most typical is to use "immortalized" cell lines. Basically, cancerous cells that can grow without bound.