it's true that many things cut down the number of upperclassmen on D1 rosters -- the draft being one of them. Some of the power schools seem to lose a significant chunk of their junior class each year to the draft. Then there are injuries, on and off the field failure, and burn-out. So you're right that it probably isn't right to think of 1/2 of D1 rosters as consisting of draft eligible juniors and seniors. But still, even if you take account of attrition, I would bet it's still true that the VAST majority of D1 players don't get drafted.
Here's a somewhat random sample -- trying to pick a range of schools -- and how many of their draft eligible players got drafted in 2015.
- Of the 21 draft eligible players at the University of Notre Dame this year exactly 1 got drafted.
- Of the 20 draft eligible players at Ohio State 4 got drafted
- Of the 17 draft eligible players at Cal State Long Beach 2 got drafted.
- Of the 9 draft eligible players at Vanderbilt, all 9 got drafted. (wow)
- Of the 13 draft eligible players at Umass Amherst, 0 got drafted.
That's 5 schools from different parts of the country, with 80 total draft eligible players, of whom 16 got drafted -- over half of them from a single power house school. If that pattern generalizes over the roughly 300 D1 schools that means on the order of 20% will get drafted.
Remember one thing though... its like the odds of rolling a three on a dice. Its 6 t o 1 of course. But if you roll it twice its 3 to 1. Not a math teacher and I am sure in statistics and probability it is much more complicated than that. But remember those draft eligible juniors have another shot as seniors! So now that 20% becomes closer to 40%! That's getting pretty large. In fact I think the 1 in 5 is pretty large. Go to baseball cube and look up 2010 rosters. The smoke has cleared there. And see how many kids eventually got drafted. Now you will have to do a little work filtering out those who quit after their freshman or sophomore season but even if you leave them in it might be in that 20 or 30% range. That's still pretty impressive. Now.if we had the means and will power to some how sort it to guys who played... like starters and pitchers who threw significant innings I bet that number would be astonishing. Fact of the matter is if you are fortunate enough to be a D1 starter your chances of being drafted are pretty darn good.