Absolutely WRONG mindset. At tryouts you are RARELY going to get a team full of starters. So you'll almost always end up with players who need work. As a coach you can not completely develop individual players in a team environment. With 12 or more kids on a roster it's impossible. What you can do as a coach is to teach them the how's, what's and why's of playing the game or a position. The players MUST work on their own to master them. If the players are "Practice Players" they will never be one of the players a coach trusts to put on the field. Often it's the parents who fail to help their child. You have no way of knowing that when you take them on the team.
Like pitching, hitting is a very individual thing. I tell my team to get individual instruction. You can only do so much at a team practice. I always set up specific hitting and pitching practices for my teams and it's still hard to watch them all at hitting.
Edit To Add: And sometimes there are kids who will never be as good as your starters no matter how hard they try. They just do not have the natural gifts.
I should not have said 'my fault' regarding a lack of development by a particular player. Coach can do only so much. I exaggerated in an attempt to describe a mindset.
As a coach, I placed a major emphasis on developing every player on the team so that each one could make a contribution in bracket and elimination games. I would be disappointed if I had a 12U team that went to the USSSA World Series and after 9 months, there were 2 of them that I couldn't let play in bracket games - pinch hit, pinch run, flex, something - because we might lose if we did. I judged myself as a coach more on those things than winning.