Time spread too thinly across too many activities is a killer for anyone truly seeking mastery. There simply isn't enough time to become a master at more than one or two things in life. Time doesn't wait forever. Health doesn't last forever. Windows of opportunity don't remain open forever. Relationships will not wait forever. Time lost is time lost. Period. Mastery is only available if given enough time, and delaying or restarting Immersion or spreading oneself too thin both deprive one of the time required for mastery.
Also, one can easily observe that attempting to compete part-time with someone who has dedicated himself full-time to a profession is likely an exercise in futility. Sooner or later the person with the most focus, the most commitment, the most "skin in the game" will win.
In order to take the fullest advantage of life immersed in a worthy pursuit, one must focus. We are all busy. There are many, many different responsibilities in life. We have family obligations, work responsibilities, friendships and other interests. However, mastery requires concentrated immersion in the subject at hand. The more concentrated our focus, the more quickly proficiency and expertise will result. That means throwing ourselves all the way in. There is no partial immersion. Immersion works because, like the Hokey-Pokey, it requires us to throw our whole selves in.