~80hrs on topic A squeezes the available time for being acquainted with the rest. [Edited because there was little way not to make the former formulation read, unwillingly, nasty]
Some of us believe instead on the advantage of being a polymath, (also) to be able to export wisdom from other contexts into the current work.
Also in terms of the proper ground to facilitate innovation.
Musk recommends 80-100hr weeks, every week
Jack Ma is a slacker...Only 72h per week.
"Jack Ma says his employees should work 12 hours a day, 6 days a week" https://twitter.com/i/events/1116787491707731968?lang=en
https://news.ycombinator.com/item?id=15251769