Your second point is fair, however I think programmers tend to stick to their "favourite programming language" like a religion and fail to appreciate that learning a new programming language can be done in 2 weeks while learning the ecosystem of libraries of a platform is a multi-year endeavour. In particular, you'll be able to carry your knowledge of all the WASM libraries you've learned to love when you decide to make a switch from say JS to XXX.
I've been writing Python for longer than I've been writing C#, but I'm nowhere near as productive in Python.
As for the second, there's zillions of things that can explain that and I'm definitely not clear that your being less productive in Python than in C# has anything to do with how much time you've spent on either. Which is the whole point, programming languages aren't equal. Neither are libraries.
If you set aside the time to learn the libraries and narrow it down only to syntax, that certainly the time to learn it will vary from person to person but isn't a multi-month task for anyone. I think you are scoping it in the larger context of non-shared libraries.
The best example I could give would be learning VB.NET when you already know C# (or vice versa).
For example, the API between your language and the other language is probably not going to match your languages style. You see it all the time with APIs that seem to be written for java but are in another language.
If the library has to write different APIs for all the different langauges, then that is an additional cost.
Then there is the fact that less people will know a "standard" language, because they will all be using different languages, so they can't contribute back to the libraries that they use so easily. For example, if I am using a scala library and want to change something or commit a bug fix or whatever, I will have to learn scala.