The difference between Python, C++, Haskell, Common Lisp, Prolog, and SQL isn’t syntax. If it was, everyone would pick their favorite syntax and use it all the time. What matters is how well the semantics (and their potential performance implications) match your problem. The syntax just needs to be a decent enough interface to the semantics. Frankly, it seems to me like most of your “counterexamples” are about language semantics, not syntax.
Here’s the thing. Would I like every language to have a consistent, beautifully designed syntax backed by UX research and testing? Absolutely. But language designers have bigger fish to fry. There’s little value in wasting energy talking about syntax once it reaches a basic state of acceptability.
I do amend my statement - you’re right that it’s a big, unsubstantiated claim. There are no _demonstrated_ differences. I haven’t seen an ounce of evidence that it makes a difference beyond familiarity. Furthermore, even if it did, that wouldn’t make it top priority. It would just make arguments about it sensible.