As I understand it, Nancy Pelosi would be acting President, until a President is chosen by the House of Representatives, if for some reason there is not a President chosen by January 20th. It’s not as if Nancy is then president for the full four year term of office.
That is not the way Laurence Tribe described it. I am not an expert on this matter. He described it as treating the election as “meaningless” if the President’s term expires and a new President has not been selected. Not my words - his.