EDIT: For those who didn't read the article, I say 5th grader here because a large part of the study is actually about them. Not because I arrogantly compare people from other fields to children.
Haven't read the entire paper but...learning from something doesn't mean you need to design something 'based' on that.
The advantage of studying how people unfamiliar with a specific way to solve problems will solve that problem is actually a great technique to try to understand common patterns and approaches humans think about problem solving.
Some pattern solving strategies will be more successful than others and it is worth understanding what is common to those (especially when compared to the unsuccessful ones)
Identifying patterns in the way people solve these problems can prove to be great design insights that go into the design of problem-solving solutions for those problems.
A lot of notation is the way it is because of history and inertia, rather than because practical considerations or requirements means it needs to be that way.
If there are changes we can make to make languages more approachable without making them worse in other ways, it makes sense to opt for making them more approachable.
Less than you may think. Leibniz and other mathematicians spent years debating notational forms in mathematics before settling on what we have. See Florian Cajori's _A History of Mathematical Notations_.
It's not their opinions. It's about how they naturally think.
And, yes, if I could change mathematical notation in a manner that made it easier to learn without introducing a more significant disadvantage, I would do it in a heartbeat.
Things are the way they are for a reason but that reason isn't necessarily that the way things are is the best.
It is a mistake and somewhat arrogant to view your domain experts as "5th graders".