Will they/them replace him/her? Probably.
It was always going to happen, and we should embrace the journey.

Though I did previously complain that we’re “bastardizing” our language with emojis, the reality is that language evolution is a natural and enriching process—and this is one we should enjoy. There has been a growing (but failing) movement to reject gender-neutral pronouns over political opinion, and the discussion over identity is well understood. But for the self-professed grammar experts who think they can block the transformation over their idea of semantics, it’s worth breaking down deeper linguistic reasons that their attempts to stop this movement shouldn’t, and won’t last long.
Clarifying grammar arguments
Before my arguments start, I’d like to debunk the wrong-way sign of grammar structure. For folks complaining that “they/them” doesn’t make sense for singular, they seem to forget or simply have no clue that it is grammatically correct, as defined by Oxford I.2.a. a1375, and has been used as a singular anaphoric pronoun in English for over 800 years. We do it without thinking—you say you’re going to hang out with your friend, someone might respond “how did you meet them?”, because we’re unaware of the gender. Even when we know, we frequently use it in place of a gendered pronoun anyway, because it’s grammatically correct. So now that we have the semantic myth out of the way…
Gendered pronouns aren’t universal, they’re language-specific concepts we’ve invented

Countless languages have no such concept. Uralic languages for example, Finnish, Hungarian, and Estonian, have no gendered terms whatsoever. Gendered terms in East-Asian languages like Japanese and Korean are almost nonexistent, what is is rarely used and with little meaning, often being used in forms of politeness more than anything else.
For the gender-less languages, they often treat a person as “it”, just like a dishwasher is “it”. It seems hard to imagine at first, but a Dutch or Italian individual learning English find our adjectives, articles, and nouns not having “genders” to be a quite rational thing.
English has been waning them out for centuries

While Germanic languages such as Dutch and German (who has three virtually incoherent genders), as well as Romance languages of French, Spanish, Italian, Portuguese, Romanian, and their sub-branches, are littered with genders, English—despite having Germanic roots and widespread French vocabulary—ironically has almost completely abandoned gender within the language. The only remaining gendered elements in English are simply pronouns, and a handful of human occupations, such as waiter/waitress.
However, English has gradually abandoned gendered occupations for entirely unrelated reasons to LGBTQIA+ movements. Terms such as “server” to replace the former, and “flight attendant” to replace steward/stewardess, and “actor” simply covering “actor/actress” have been replacing their predecessors for decades.
The formers actually begun over simply revising our vocabulary to remove negative connotations that slowly crept into their meanings, while the latter (actor) made its way in for ease and simplicity to the point it is widely accepted, and will undoubtedly be the standard. Both The Observer and The Guardian have already updated their official styleguides to reflect “actor” be used in all cases, except for official awards such as “best actress.” They noted that the term “actress” is all but obsolete, following the footsteps of authoress, comedienne, songstress, manageress, and lady doctor, from times when professions were usually specific to one gender. When is the last time you heard someone say “songstress”? I bet many of you didn’t even know that was a thing. Case in point.
Historic language evolution—the rise of simplicity.
We’ll take French as a prime example. Despite its obnoxiously complex rules, over the centuries French vocabulary has abandoned vowel and consonant pronunciation to a frequently comical state. While plural and singular pronouns such as he/she (il/ils, elle/elles) have almost completely dropped their S pronunciation (the remaining exceptions as spoken liaisons an ironic need to maintain spoken clarity), it has also gradually replaced the word “we” such as “nous devons manger/we must eat” with “one” “on doit manger/one must eat”. The reason? Simplicity. The “nous” conjugations are long and annoying. And nobody officially facilitated that, it’s just our evolution towards that simplicity.
It’s called evolution.
This goes on largely unnoticed, and relatively fast
What’s most remarkable is that these changes haven’t taken long, they happen in a few short decades. In the 80's, my mother was a stewardess. By the beginning of the 2010's, “flight attendant” had already replaced it entirely. When I worked in a restaurant, I heard “waiter” less and less by the year. Nobody can quite pinpoint exactly when “on” essentially replaced “nous” in French, but older generations will tell you that it was much less common in their heyday. Many institutions are already accepting it is grammatically correct (although the hilariously and increasingly disliked Académie Française refuses to acknowledge it—the same organization who demands people call streamers “joueur-animateur en direct” and tried to force replacing the word WiFi with “l’access sans fil à internet”).
It was bound to happen, no matter the case.
For they/them pronouns, I would join the argument that it was an inevitable transformation. For one, there’s no real confusion in identity when someone refers to me as “they,” just like gradually calling actresses “actors” also didn’t confuse anyone. Languages naturally evolve to lose archaic structures: simplicity is a natural propeller, and English has been at the forefront of making gendered vocabulary obsolete on its own for centuries. Just as no English-native would complain that we don’t have gendered versions of words like “door” or the concept of “flatness”, it’s worth looking forward to the next step of simplicity. Seeing that this has been a constant wheel for its entire history, the gradual abandonment of “he/she” was already making its way in, and I would argue that even if personal gender identity was not a concept to begin with, the road was already leading there. The fact that individuality has propelled this change is a wonderful thing. Attempts to stop it are futile, just as they have been for every other case of language evolution. We should acknowledge the natural journey, embrace the transformation, and stop wasting our time trying to stop the moving train to hold onto antiquity. Or if I was to say it as if our language never evolved, thou all hast been pareth of thy journey, et it shall befall with, ‘r without thee.