I tend to think, of something like "the first <foo> to <bar> in the whole <tump-and-tumpty-tump> years of a insititution's history" something like "Great, but doesn't that mean more about past exclusionary practices[1], and it never guarantees further breakthroughs.
Mrs Thatcher was hardly a poster'girl' for female PMs. She did not (by inclination and/or by the limitations of the available pool of possible 'talent') have much diversity in her cabinet ('the vegetables'), certainly no direct encouragement for "the next female PM"[2] to come about.
Similarly, those who hated Obama aren't going to quickly support "the Republican Obama", whichever one that may be. If Hillary had become POTUS, it probably would not have helped womankind much (after the fact, even if she helped during).
It
might be helpful for <insert a different unprecidented president here> to have "already had a black guy", etc, so maybe someone openly gay/whatever could take their own Great Glass Elevator ride to the top, as being more novel than "yet another of what we just had".
And fit all that into
https://xkcd.com/1122/ and
https://xkcd.com/1122/ if you like!
[1] Implicit or explicit. It could have been that women (say) were barred from a given position, or just that there were no women given enough credence by the lower parts of the structural hierarchy (e.g. never even being apprenticable to that trade, never being free from home-duties to get involved in studies, or being barred from direct involvement in politics as a side-effect to a rule about personal wealth that was really the fault of a misongynistic marriage/inheritance-law) that meant that there was never going to be a plausible candidate (for Guild Master, Chancellor Of The University or Leader Of The Party, to continue those themes) even
before any willingness of the nearly-top-men to appoint a woman to the toppest-of-the-top job becomes a factor.
[2] Whether May got a boost to her chances for not being the first, or even took heart at Maggie having at least carefully cut a hole in the glass ceiling, doesn't stop us realising that there were no high-ranking women in UK politics (already a low number, with a number of competant candidates) who achieved the same feat so long as Mrs T was alive. Though May's somewhat dithered and issue-troubled Premiership shows it
not to be the case, the fear of another Iron Lady (from all sides of the political spectrum) might have worked against all potential successors.