The union no doubt would have great benefits. I have a hard time accepting the fact that a business owner would throw money at a guy without any experience at the big league level. That is like me going for a job as a Surgeon and getting a sign on bonus and big salary because I have potential and have some experience cutting a steak.
It's not remotely like that at all. They're so far past the "steak cutting" phase once they reach the MLB.
But, you're also not realizing the context as it relates to MLB. Entry level salary of 465K (which is pro rated by the # of games they play) is small when the elite of the sport are earning 25M. That's only 1.86% of what the top makes, and about 25% of the median.
And, as AG pointed out, they're pretty vetted by the time they get to the MLB.