Umm... you realize that shit needs to be paid for _somehow_, right?
Having a larger more educated workforce should pay off in the long run. If the costs are insanely high like current private college costs then maybe you're right though.
The benefits to the community of having a higher educated population are something like 5x the tuition fees. Just the tax rake from higher educated professionals more than pays for free tuition.
But this isnt an economic argument - its about how can we ensure that we can continue to have a lower class to do all the dirty work.
I remember reading about one of the big pro chattel slavery arguments and it was that the economy of the South would completely crash without slave labor and the whole nation and maybe world would suffer. Once they were freed and became paid sharecroppers the crop outputs were higher than ever before. Turns out cheap labor isnt very motivated to do a good job.
Not long after this the absence of free labor pushed market forces to automate most farming jobs. Slavery(the ultimate cheap labor) probably slowed down the rise of automation in agriculture.