Hopefully I'll live the couple of decades to find out if PG's prediction is correct, I would bet against it.
Here however, I do agree with his articulation -- "writing is thinking" -- and like you, I've thought a bit about the linear nature of writing.
My view is that the "jumble" of ideas/concepts/perspectives is just that -- a jumbled mess -- and the process of linearizing that mess requires certain cognitive aspects that we (humans) generally consider as constituting intelligence. IMO, the rapid generation of grammatically-correct + coherent linear sequences by LLMs is one reason some folks ascribe "intelligence" to them.
I liked his analogy about how the disappearance of widespread physical work meant that one now had to intentionally invest Time and Effort (at the gym) to maintain physical health. The facile nature of LLMs' "spitting out a linear sequence of words" will mean fewer and fewer people will continue to exercise the mental muscles to do that linearization on their own (unassisted by AI), and consequently, will experience widespread atrophy thereof.
I suspect thinking is similar, which brings up questions about LLMs as well. We all can now quickly write hundreds of generic business plans, but knowing what to focus on first is still the hard part.