Labor unions are a classic example of, just because a little is good, it doesn't mean more is better, and too much is just enough. When Hoffa first started to organize the truck drivers in the 20's and 30's and created the Teamsters Union, his intentions were good. And at first he succeeded in making truck driving a non abusive, honorable profession. Same with the first guys in the Industrial Revolution who rose up and organized workers against the deplorable conditions Andrew Carnegie created in his steel mills.
Then, as with most labor unions, when they grew to enormous membership, wealth, and power, in came the mob, along with a list of assorted other no goods, leeches, and greedy politicians, and everything started going to hell. Wages and benefits got so high, along with companies all but powerless to fire bad workers, the laziness increased to the point of a lot of these guys becoming all but worthless. This drove up prices of products until union companies could no longer be competitive. Look at the steel industry in and around Gary, Indiana if you need an example of what happens when unions get so big they destroy jobs, instead of make them better.
You're right in that people are finally seeing how totally worthless they are making people like teachers become.