[jimsug]

Well, that's not entirely true either.

> The driver's coming at 5pm and they're always on time, so we'll need to be ready for them.

Seems pretty unambiguously alright to use it in the singular there, unless there's more than one driver

[humanrebar]

1. It's not clear from your snippet if the vehicle will arrive empty or with passengers.

2. Even if that sentence is unambiguous, it is not hard to come up with sentences that are ambiguous.

3. Colloquially people do all sorts of imprecise and grammatically inadvisable things like "Give 'em a hand." That does not mean the phrasing makes sense for formal, professional, or technical writing.

4. The proof of the pudding is in the eating. If people are confused by singular "them" (they are, but someone please do a proper study to confirm empirically), the construct is confusing.

5. If an author feels it is ambiguous in some cases, how do we accommodate that without branding the author insensitive or even hateful? It seems also unwelcoming to not leave room for agency in grammatical decisions.

[jimsug]

Studies around singular they/them:

[1] it was in use long before the prescriptivist grammar movement (yes, I know you've raised that this has shifted but I just wanted to highlight that this is not a novel use): https://pdfs.semanticscholar.org/328b/81dbcbeba4bf5c7210de59...

[2] If the gender of the antecedent/referent is not known, using singular they/them speeds up comprehension. If gender is known, it slows it down compared to using the known gender pronoun: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4293036/

[3] Some nice examples of less ambiguous referents: https://www.tandfonline.com/doi/pdf/10.1080/00437956.1993.11...

To address your points, though:

1. Not sure where these passengers came from, but sure, if you can insert entities into the referent then any pronoun is potentially invalid.

2. Of course you can, but (a) context will fill in the gaps and (b) if that doesn't work, you can usually interrogate the author/speaker (and (c), if you can't interrogate the author/speaker, then yes, they've failed in their composition - but that doesn't necessarily mean that singular they/them is the problem, or is infeasible in all contexts.

3. Sure, there are many things that are acceptable colloquially and not professionally, and vice versa. Does writing on an internet forum fall more in line with formal or informal writing? I don't think it's clear.

4. See the above; sometimes it "confuses" people, sometimes it doesn't (and the unqualified assumption that it does is perhaps an indication that there's some motivated reasoning at play)

5. I don't myself have any issues with writing to avoid pronouns, however if you pointedly avoid pronouns with one person and then use them with another (let's say you use them with a cis-female and then avoid them with a non-binary person) that's not really welcoming or sensitive, and it seems apt to brand them as such. If you're avoiding pronouns equally in all situations, that's different. But if you're selectively avoiding pronouns where they don't line up with your personal viewpoints, then you might be seen as insensitive or hateful, and you might not be welcome everywhere. That's kind of how life is, and you just need to deal with it.

[jimsug]

Whoops: [2] indicates that using singular they is as good as 'he' for male referents, and better than 'she' for female referents.