If 0bama is shown to be ineligible, McCain would continue to be president. After all, McCain had the highest number of electoral votes among the eligible candidates.
Beg to differ.
The 25th Amendment reiterates what is stated in Article 2, Section 1: that the Vice President is the direct successor of the President. He or she will become President if the President cannot serve for whatever reason.
Moreover, even if Obama is shown to be ineligible, Biden was still elected to the office of Vice-President. McCain was elected to nothing.