For all we know, animals could have been trying to teach us their languages for centuries

AFAIK it's only very recently that we began to try and learn the languages of animals rather than teach them

Parent is arguing that they need to witness language use between members of their own species, and then not learn, in order to show they are incapable of language use.

The first human language user managed without this. (Though, if used for communication, it takes two... so there couldn't have been a first. OTOH there's an argument that language is primarily for thought, not communication, so it only takes one person. Secondly, evolution doesn't usually build a complete perfect system, but iterates, gradually improving. A puzzle here is that abstraction seems to be all or nothing, you can't have part abstraction. One resolution is that the necessary neuro-machinary was built up for other purposes, and abstraction emerged when some crucial capstone happened to be put into place. Or... maybe you can have some kind of "partial" abstraction...?)

I think you could have partial abstraction. People move around physical objects which represent other things, all the time.

Maybe representation is more important than grammar? e.g. representational art

The bee-dance "represents" direction and distance, but only that.

I wonder if it's a matter of skill or desire

Or, evolutionary advantage (primarily in the meme sense, but also genetically, for improving physiological and neurological support for language).

In what contexts does abstract language confer a survival or reproductive advantage, beyond simple communication, which is a commonplace, e.g. predator warning, food location, mate-seeking.

That’s our failure. Not theirs.

One huge failure is thinking that animals should care to learn anything we teach them.

If animals are unable to direct their attention towards learning to do something, that's functionally identitical to them not being able to do it. Even in an alternate universe where the only reason dogs can't speak English is that they can't work up the focus to sit down with a copy of Strunk and White, it would still be correct to say that animals lack the capacity for language.

You're assuming that not doing something implies an inability.

Does the fact that I have not adopted your position imply that I am unable to do so?

Why can't I be unwilling?