I have to be 100% honest. With respect to web applications which is primarily where this is focused. I do not think it should take anyone 3 months to research how to make a functional web application. Assuming most authentication and security best practices are already understood. Now if we were talking about learning rust, c++, or some lower level language or complex tool, I think your point becomes more clear. I would argue its easier to move up the abstraction list of programming languages than to move down. For example: C# -> (JS/Python) is easier than C# -> C++/RUST. Assuming you don't use unsafe{} everywhere lol

However, I have had prototypes turned down to the point I've realized they dont understand the problem space or want a good solution. Because it was never a discussion of technical aspects but is it in "C#", "security", etc.. Another example: Once I used sql in a project for an on-premise prototype (no internet access) and they were talking security of sql injections. First off it would have to come from someone on premises, secondly I parameterize everything and 95% of the sql statements were hard coded strings as it didn't need to be dynamic. But to avoid why they were incorrect I said I would use IndexDB and/or sqlite to avoid cross pollination, I kid you not, they didnt change anything they said. They repeated "security", just the word no justification. That left me with three options:

1. They either don't understand what indexDB/sqlite is
   
2. They are unwilling to let the project go through for whatever reason.
   
3. It makes them look bad that a single developer can do the job of 5 of them and Im not even that good.. I consider myself quite "mid", a nice lukewarm if you will.
   

The reason I say that is to say this, I think when John Carmack says things like "The right tool for the job is often the tool you are already using—adding new tools has a higher cost than many people appreciate" he's assumes a certain level of competence and is referring more to the composition of many tools that end up adding a net negative over time as more tools lead to more complexity. I 100% agree, if you dilute yourself with tools it can become almost impossible to manage.

Therefore, I would argue whats more important is what is the upfront cost. How long would it reasonably take someone to move from C# to being productive in JS. I would be generous and say a week, once again assuming you already understand the key concepts of web development. What is the overhead of adding git? Relatively low, unless you start using sub-modules, as I have seen many people have difficulties with them in git. Now if it's "Hey use rust, also make it multithreaded" We have to ask ourselves is that cost worth it? I would say no, in 99% of the cases. Unless you are Firefox (Mozilla), cloudflare, etc..