Here is a thought, just a thought. Suppose we really did have actors playing Joe Biden for several years while his administration, Elizabeth Warren, and Obama ran the government. Well you can't continue the charade. The actors need to stop and find another gig, so they would have to have him die quickly.
Elizabeth Warren said last July that "she had not seen any decline in Joe Biden over the four years". That indicates that Joe Biden might have died four years ago. Because the guy asking her the question said that she had said "he was smart as a tack" and she couldn't control her urge to laugh. That tells you the entire four years were a farce, dead or alive, he was incapable of being president. So in reality he could have died at any time. However, if he had died two years into the term her claim that "she had not seen any decline" would be false.
To impersonate a president runs a very high risk of being caught. She doesn't want to lie knowing full well the day may come where she will be held accountable.