Update — such a discussion is a bit academic. I don’t always have a choice to focus on one area. I can’t afford to focus too much. Many domains are very niche and there are very few jobs.
If you choose the specialist route instead of the manager route, then you may find many of the successful role models need focus and accumulation. An individual’s laser energy is a scare resource. Most people can’t focus on multiple things, but look at Hu Kun!
eg: I think many but not all the traders I know focus for a few years on an asset class to develop insight, knowledge, … Some do switch to other asset classes though.
eg: I feel Sun L got to focus on trading strategies….
eg: my dad
All the examples I can think of fall into a few professions – medical, scientific, research, academic, quant, trading, risk management, technology.
By contrast, in the “non-specialist” domains focus and accumulation may not be important. Many role models in the non-specialist domains do not need focus. Because focus+accumulation requires discipline, most people would not accumulate. “Rolling stone gathers no moss” is not a problem in the non-specialist domains.
I have chosen the specialist route, but it takes discipline, energy, foresight … to achieve the focus. I’m not a natural. That’s why I chose to take on full time “engagements” in c#, c++ and UChicago program. Without these, I would probably self-teach these same subjects on the side line while holding a full time java job, and juggling the balls of parenting, exercise, family outings, property investment, retirement planning, home maintenance…. It would be tough to sustain the focus. I would end up with some half-baked understanding. I might lose it due to lack of use.
In my later career, I might choose a research/teaching domain. I think I’m reasonably good at accumulation.
 home maintenance will take up a lot more time in the US context. See Also
https://1330152open.wordpress.com/2015/08/22/stickyspare-time-allocation-history/ — spare time allocation