It seems mixing cause and effects in many cases. It could also be that most successful people focus on only one thing because they are successful they can just afford to do only thing and reap exponential rewards. For people like me in mediocre career / jobs in mature industries, doing best at job just means writing even more half-assed CRUD micro services.

The best possible thing that could happen by totally dedicated to job would at best be one more promotion to keep working on same mediocre stuff or manage a team doing that for few thousand dollars more. And it is not even blaming management as there are only limited opportunities in saturated industry and far more folks are chasing them.

Even those half finished projects have given me more knowledge and experience than mind numbing "next generation technology" training that corporate got a great deal on by some discount instructors.

Now it could all have been different if I joined a hyper-growth industry at right time. I might have gone far by just knowing one thing. At that point I could be proselytizing younger people that "you've got to focus on just one thing and you will make it big"

I saw exact example of this by listening to a SVP in my office who apparently rose through ranks by "sheer hard work, grit and focus". However to me it was pretty clear he joined company just at the time when industry as a whole about to enter massive growth. Of course he isn't gonna say how stars were aligned and he just got massively lucky. It is for others to see what it really was.