(1) It's unlikely that you will be the person who knows the most about any subject. There will always be at least one person who will know more about any given thing--unless that thing is defined so narrowly that nobody actually cares. You don't really have to worry about that, because you can be second best (or tenth) and still make a contribution. There's a law of diminishing returns, too. Often the specialist on an author will no biographical information that nobody really even needs to know.
(2) It is relatively easy to gain a respectable amount of expertise about something. Say someone wanted to learn about Dickens. It would require a lot of time to do the reading, but anyone, in principle, could just do that reading. Read all the novels, then read the criticism until you pick up a book that doesn't tell you anything you don't already know. (This for fields in which there is no huge technical barrier to understanding.)
(3) Expertise does not mean originality. You know Dickens, you know what everyone thinks about Dickens. Maybe you think the same things as everyone else, more or less. Originality comes from the encounter between you, a unique individual, and the text. The text strikes you differently, it takes a funny bounce when it hits your mind, the same funny bounce every time. You start to study that bounce and develop an argument from there. Maybe your expertise is only respectable, not extraordinary, but you can still make a contribution.
(4) Then "all of a sudden" after 10 or 15 years you will be one of top experts. It still doesn't matter if there are few who know more in absolute terms, if you know almost as much but are a little more original.