The more you learn, the faster you can learn

Suppose any rate of decay in the things you retain. If your starting point is already small, losing any of it comes at a high cost. At least when pursuing mastery of a subject, you are repeatedly faced with new ways of understanding the basics, to the point that they are innate and automatic. You might forget small specifics which you can later find in your known references, but without mastery you might instead forget the basics--and relying on a reference for those basics means it's much less clear when they are aptly applied.

I would hazard a guess that that's why people have always tended to master only one or a couple skills, but people congregate and work in complement with others who have mastery over other skills. The community is the entity with knowledge over many domains--not the individual who learns tons of surface material. And simultaneously, everyone does learn a little bit about a lot of topics from time to time. But that balance is important--so I wouldn't personally weight so heavily the cursory synthesis portion of it as OP did.

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Also, a little advice to OP. Drop some of the grandiose introduction. 100% of the links I clicked in the first paragraph were more disappointing than their descriptions led me to anticipate.

Syntax is a very minor piece of the programming language pie. If Python, Haskell, and Rust all shared the same syntax an expert Python programmer would still have a hell of a time getting anything done in either of the other two languages.

That being said, I do think the importance of mastering one or two things should not be overstated, and was kind of ignored in your post. Let me elaborate: for me my deepest skill has been 3D modeling and rendering: it's been about six years of pretty consistent practice, doing motion graphics, general modeling, animation, color theory, and really just trying to absorb all the knowledge I can about this specific passion. This process has been super useful for a couple reasons, and kind of worked to snap me out of the "generalist" learning mindset. For example, I've found the learning curve over time for 3D modeling is pretty generalizable to other skills I learn: because I've sunk so much time into it, and have had to push through plateaus, and grind over hills, and realize when I'm just coasting, whenever I am learning other things, I recognize the emotion swing and know roughly where in the learning process I am. Similarly, the fluency I have with my 3D modeling tools (pretty much all the hotkeys burned into muscle memory) gives me something to strive for when learning other tools and skills. I know what fluency feels like now, and I know when I'm lacking in it.

For me though, the most important thing about pursuing mastery of one or two things instead of full generalization, is that it teaches you patience. Genuine patience. Something I feel is becoming a lost art in our modern time. Seeing how far you've come after six years of working at something, and then looking at your inspirations and seeing how far you still have to go, is a feeling that can either break you, and make you give up, (if you are impatient), or can help you, as you have the choice to just keep grinding away and choosing to be patient. 3D modeling and animation is a ten year (minimum) goal for me, and it's served as a wonderful learning experience for me in many, unforseeable ways.

To tie it back to your post, I take this "ten years" mentality to almost everything I choose to learn nowadays: I try to stay away from the fancy new frameworks and toys, and try to learn fundamental things that I can spend ten years learning and still have space to grow.