> I agree with you: learning a new language, sometimes in the heat of the moment without even knowing the language existed before, is a thing skilled software developers are able to do remarkably quickly
Maybe it is true, for some people and for some languages.
However, learning the syntax of a new language means nothing if you don't already learn libraries, tools, frameworks, tools, patterns, idioms. And that still takes time.
Memory isn't unlimited and while you learn new concepts, try to make room for new concepts, you start to forget some of the old concepts, especially if you start using them less frequently or seldom use them.
So, what I try to teach in my Comparative Machine Language Morphology course is the skill of being able to go from nothing to expert in a technology almost immediately by trying to get your brain to generalize into what we know about how humans conceptualize algorithms.
To tell a quick anecdote: when Apple announced Swift, I was hearing about it for the first time in the audience at WWDC during the keynote. As I watched, it was quite clear to me that this was a language that was mixing parts of Scala, Haskell, and (awkwardly enough) Ruby with the background of Objective-C, and it just immediately clicked. I ended up giving a talk about the language a few days later at AltConf.
The reality is that, unless people go extremely far out of their way to push their language in a way that makes it awkward for the user--I had a student who designed an esolang based on gravity and "roadrunner physics" where the code was a map where things didn't start to fall until they had some kind of interaction--there is very little "new" under the sun: the vast majority of "advances" in programming languages were already pioneered many decades ago.
(The most ridiculously-obvious example of this is probably Go, which has almost nothing that wasn't already understood by the 1970s, including its mechanisms around channel-oriented concurrency. There is an amazing article that attempts to compare Go to a "Brand X" language that turns out to be Algol 68. But like, Rust's entire schtick--the reason it is called Rust!--is because they actively refused to do any new language research and instead based it all on well-worn concepts, and yet people still whine about it incessantly merely because they haven't put anywhere near enough time into learning about the history of programming.)
Maybe it is true, for some people and for some languages.
However, learning the syntax of a new language means nothing if you don't already learn libraries, tools, frameworks, tools, patterns, idioms. And that still takes time.
Memory isn't unlimited and while you learn new concepts, try to make room for new concepts, you start to forget some of the old concepts, especially if you start using them less frequently or seldom use them.