The fallacy in his argument is that it assumes that there is somewhere, somehow, a huge pool of untapped talent who are in line and are good enough to play.
Nothing could be further from the truth. MLB actions demonstrate the fallacy; every club has a large number of people devoted to scouring the country looking into every nook and cranny for what is perceived as "potential." If the clubs could simply put out a "help wanted" sign for companion ponies for the thoroughbreds, and that action produced those companions, clubs would do it (because of the huge cost savings). The fact that the overwhelming number of those who make it are from the first 10 rounds does nothing to prove that the remaining draftees (and the few FAs signed after the draft) match the talent of this huge pool of equally talented (yet undrafted) companion ponies.
Look at the draft; look at the guys not drafted. The best hitters, best fielders, best pitchers, best players (either because of their actual production or because of a scout's ability to pierce the kids present production and peer into the future development) are the ones taken. While I have seen parents, players, coaches, and assorted pundits claim that a player drafted was inferior to one who wasn't, I have NEVER heard the scout who was responsible for drafting that player agree (except in rare economic situations) with that assessment (at the time of the draft) - the clubs might draft a lesser "player" in a few instances due to economics, but the club sees something in the player (otherwise why spend the time and money in scouting that player).
As for the argument that the players are "apprentices," I would urge those who believe that claim to read the definition of the narrow categories exempt from minimum wage. The MILB players do not fall in that category (indeed, these players are, with the exception of those higher in the chain, amongst the most skilled in baseball in the world - before they even get drafted).
Over time, we have been over the "they knew what they we getting into" assertion. That is a smoke screen; Public Policy (society's determination that an employer must pay a minimum wage) trumps an employee's ability to accept - and an employer's ability to pay - less then society has deemd as a minimum (with narrow exemptions which are at the core of MLB's defense).
Apart from the law, MLB has chosen (IMO) to short-sightedly place immediate cost savings (expressed by the low wage) over long term develoment which could be enhanced through better nutrition, better personal physical training in the off season, etc. I have no ability to quantify or research whether there would be any long term difference if players were able to eat and train better; the reason for this inability is that EVERY CLUB MUST adhere to the same wage rule. It's too bad that even if a club wanted to pay its milb players minimum wage, no club would be allowed to do it.