Re: Sheryl Sandberg was/is considering a run for President
> how many past holders of that post (never mind the present one) have been qualified?
Pretty much every single one starting with FDR. I may not agree with their politics or policies but none of them appeared to be in completely over their heads when they declared their candidacy.
Yes, Nixon did terrible and stupid things, but he was Vice President before being elected. JFK and LBJ were in the Senate. Carter was ineffective once elected. Reagan was Governor of California before being elected. All of them - with the exception of the current one - had demonstrable experience and qualifications in running a government, or working for and with the government, with some degree of competence.
I'm including W. in that category. Yes, he made terrible mistakes while in office.
Getting elected to office != qualified to hold the office.