Deep vs Broad Learning
We've all heard the 10,000 hour rule to becoming expert at something. But there is also another aspect. Critiques of the 10,000 hour rule have pointed out that experts learn their skills while trying other things. For example learning to play basketball, wresting, and football will eventually make for a better baseball player than a player who just stuck to baseball from the beginning.
The same with music. The students that can play multiple instruments, can sing well, and sings in a choir is set up succeed better than the student who focuses only on piano from age 6.
One book, I can't remember which, points out that experts go through a sampling phase where they try lots of things tangential to the main task which they eventually become expert.
What I've seen from the best students is that they are eager to try lots of different things. They will play all varieties of music.
I try to always have students work on easier pieces they can get through fast as well as work on harder pieces that might take them a month or longer to learn.
Quantity leads to quality