We've had recovering-alcoholic presidents (USA), but it'd be very difficult to see a trans president anytime soon. The intensity of ostracization is not at the same level.
(e.g. https://en.wikipedia.org/wiki/George_W._Bush#Alcohol_abuse )
(e.g. https://en.wikipedia.org/wiki/George_W._Bush#Alcohol_abuse )
People would have said the same thing about the likelihood of having a gay president 15-20 years ago. Instead, we had an openly gay and married presidential candidate, Pete Buttigieg, do quite well in the last election with his homosexuality not being a major point of the conversation.
The amount of progress on gay rights and the public's perspective on LGBTQ rights has been extremely fast. I think (and hope) that you are wrong and that society will be ready for a trans president much sooner than you anticipate.
It will be a while before a trans president is a realistic prospect. The U.S. have yet to elect a female leader of the country 40 years after that was a thing in the UK, and 20 years after Germany. The U.S. now have an openly gay secretary of transportation, hardly a key position in the administration. Germany had their openly gay vice-chancellor and secretary of state a decade ago.
I think society will be ready long before the statistical chance of an otherwise front-runner happening to also be transgender is likely to be satisfied.