It's because you didn't understand it

> 'how can a cancer cell live indefinitely but other cells cannot'

Cancer cells are damaged cells mutating without regard for function. It's pretty obvious that there is a difference between "living indefinitely in a mutated form devoid of original function" is different from a cell performing a specific function

No

> or do you believe that it's impossible for any animal whatsoever to have a long lifespan

Let's pick an arbitrary timeframe and declare that as long

No, I don't think anything is impossible when you pick dates "for the sake of argument"