College baseball is hard and not everyone is cut out for it. Many think they are, but once they get into the grind, they drop out. (see below) Parents should expect this just by looking at the statistics. (I know I did not until I saw it)
Baseball ends for everybody at some point. Very few are able to play in college, and significantly fewer stick it out for 4 years. (like 75% of those who start drop before they are Sr's.))
Do not underestimate injuries, there are lots of them and many lose part or all of a season. (mine did)
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Been doing some roster research at my son's school the last couple of weeks, just to get a more realistic picture of what might be in store for him. Your point about attrition along the way seems to me to be very well taken. Just as an example, at my son's school over the past several years on the order of 9 or 10 non-senior players each year who are on the roster at year x are not on the roster at year x + 1. That's roughly a1/3 of the roster. The number of seniors each year is somewhat inflated by a steady but small stream of JC transfers. Which I found a little surprising for a D3.
Not sure how much that generalizes or if it varies from level to level Couldn't say whether the attrition is more or less severe at D1's as opposed to D3'. Of course, D1's, at least top ones, tend to have a lot fewer seniors because many juniors get drafted. But I'm willing to believe that independently of the draft, attrition is high at any level.
Not saying attrition is necessarily a bad thing either. I know that when I myself stopped wrestling at the beginning of my junior season in college -- something I had been doing at a high level through junior high, high school, summer AAU tournaments, and for two years in college -- it actually felt like a great weight had finally been lifted -- even though I loved wrestling -- and that I was actually for the first time an actual full out college student. I imagine many give up baseball in college with the same sort of feeling.