"If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President. If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President elect shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified."
Source: Article III, 20th Amendment to the Constitution of the United States.
That quote proves my point. The only Constitutionally provided ways for a VP to become President are the death of the President or the Impeachment and Conviction of the President. There is no provision for a President to be ruled as not qualified, only a President-elect.
Thank you for the clarity in your response quoting the 20th Amendment Section 3.
Thank God the Senators in the ‘30s saw fit to amend the Constitution.