Though yes, as you noted, brain plasticity would likely compensate, at least somewhat. As I've grown older, I've increasingly noticed that there are many people who do seem to have fundamental deficits in organised thinking and linguistic abilities. More often written, but also spoken. Sometimes as elements of another condition (e.g., Parkinsons), but in many cases apparently either congenital or aquired through early environment.
Brain capabilities probably vary as much as other phenotype characteristics. No one without the genes for it will be able to do a good still rings gymnastic routine. And at the other end of performance, many will struggle to finish a 5K. But the fact that you can't run well doesn't mean that you can't shot put or swim well. The use of IQ as a single dimension obscures a multiplicity of abilities.
I've wrestled with (and mostly fought against) single metrics of quality for most of my career, though being able to articulate what the problems are has only come to me relatively recently.
Quality is ultimately suitedness to task, and tasks vary. The more specialised a task, the narrower the quality component parameters, much of which may come from disabling rather than enabling characteristics. Anna Karenina / rocket-science principle: there are many ways to get a complex thing wrong, generally only one narrow path to getting them right.
The breadth of evolutionary adaptations displayed by life on Earth, or even within the narrower scope of Olympic athlete body morphology, is an instructive lesson. None is globally "better", each is adapted (or selected) to purpose or goal.
Though yes, as you noted, brain plasticity would likely compensate, at least somewhat. As I've grown older, I've increasingly noticed that there are many people who do seem to have fundamental deficits in organised thinking and linguistic abilities. More often written, but also spoken. Sometimes as elements of another condition (e.g., Parkinsons), but in many cases apparently either congenital or aquired through early environment.