Why is it hard to argue lifespan has been a major factor?

You speak of “prerequisite brain structures to support recursive grammars”… would they arise in short lived species, given we know these are complex brain regions that take time to develop, need experience to become fully useful, and are energetically expensive to maintain and run? How do you do that if you live a relatively short time?

On the flip side, precisely in longer lived species like whales and elephants, your statements begin to fail. Elephants (when left in the wild) do accumulate knowledge of huge stretches of a continent and pass on that knowledge horizontally and also down to their progeny.

Whales and dolphins also show initial signs of such linguistic competence.

What these species don’t have is our opposable thumbs, etc. Things that aren’t directly responsible for language and would have evolved without it (as opposable thumbs have in other species), that nevertheless added to what language can do.

That should give you a sense of how language in humans is only as complex as the humans and their society is.

As for writing, I’m really not clear that it’s better than leaving smell traces, when it comes to information density, and speed of sharing. Where it does better is in surviving across generations, and that is indeed an advantage, that our language can go through a short lived medium (sound) AND a longer lived medium which doesn’t tax our memories as much.

Note that writing is a much later invention though, than language. And very tied to our opposable thumbs which can make fine marks and do so repeatably enough that we could associate repeated sounds with repeatable symbols and generate this writing based transmission system. Take that away and we’d remain an oral culture. We’d have do what Sanskrit users did before they found stable ways to write, and create ways to improve memorization. That too has survived generations. Just takes a lot more of each person learning, compared to writing.