Some players do retire with money left on the table. Some people also win the lottery, get eaten by sharks, or hit by lightning. That info doesn't convince me to buy lottery tickets weekly, or avoid swimming in the ocean or going out in the rain. The Sox are overwhelmingly likely to get stuck with that cap hit, and we should prepare for that and be surprised and happy if it doesn't happen somehow.
Also, I looked up some more info on the minor leagues. It's hard to get detailed info, but it looks like teams do cover health care premiums for players, but I'm not sure if that is only in season, or what happens if a player gets injured and they release him. There is also a minor league pension plan, but it's so hard to find info on it that most minor league players don't even know how it works. According to the video below, in the plan from 2008, you need at least 5 years of service, and you get paid a certain amount a month per year you spent on each level. In the example he gave, a player who spent 2 years in A ball and 3 in AA would get a pension of $82 a month. Players who made it to AAA would get an extra $22 a month for each year they spent there. So if you played for 6 seasons and made it all the way to AAA, you'd get about $100 a month.