What I wrote does not preclude ditches being dug, or being a ditch digger. What it does is prevent ditch digging from being one of the few options for many people which result in ditch digging labor prices to be very low.

You give people opportunities to do many more things than menial labor, then that allows the price for menial labor to rise so that it is not done by the “lower” socioeconomic rungs.

This is a good argument, IMO. The main dangers I see are of status-signaling "non-skills" dominating education, and of overall malinvestment in skills that aren't needed.