“It is a mistake to think that that little room has elastic walls and can distend to any extent. Depend upon it there comes a time when for every addition of knowledge you forget something that you knew before. It is of the highest importance, therefore, not to have useless facts elbowing out the useful ones."

I think that last paragraph would be a mistake - as a person with enough self importance to think they are even a little bit of an expert in some field would stop studying others. Knowledge evolves, expands from analogs in other disciplines, etc. Dunning Krueger would stop a person in their tracks following this logic.