A very good point. A retro mode would be designed to awaken one's sense of nostalgia, and the younger fans are less likely to have experience with 16-bit baseball.
Retro mode is absolutely for us 30-somethings and 90s kids.
It's also an arcade mode, which simplifies the game for those casual baseball fans or fans of arcade action. That being said, if you are a casual baseball fan, RBI Baseball will be cheaper. If you aren't interested in sim baseball, why would you spend $60 on a sim baseball game only for its arcade mode?
Perhaps, then, it acts like a gateway. I've been playing arcade/retro mode and learned the rules and got the hang of it, and I have the sim part right here. Might as well go to the next level.
Other than that, the retro mode seems like it's for fans of sim baseball who are otherwise nostalgic.