The OP includes "And before someone says it, simply working games does not keep an umpire sharp. If anything, that is where the umpire tends to become lazy especially toward the end of a long season. All of a sudden they are working a playoff game and someone points out a few flaws of which the umpire wasn't aware."
My sig certainly agrees with not just working. Feedback and critiques are needed, along with the refresher instruction Mike is promoting. However, actual umpiring does keep us sharp and rust sets in quickly. Also, umpiring other sanctions keeps the basics sharp, even if the differences need review going from one to the other.
Don't know where the lazy part comes from, don't think I get lazy, but maybe because of just weekends after school season.