Not drafting highschoolers will not have a sizable effect on the quantity of players going to play college ball, but those players are generally of a higher quality.
The most recent stats I found were from the 2023 draft. There were 113 highschoolers drafted out of 614 picks that year (I didn’t see any stats for those who were drafted, but didn’t sign and went to play college ball instead). I’m guessing a top-tier program (read, in part, willing to pay the most) would be doing great if they got four of those players. Yes, that will push out to different programs lesser quality guys whose spot is being taken at the top tier level. Or maybe some go juco to be draft eligible after one year (unless that rule is changed).
MLB is shifting player development costs to college programs (and their fans), while probably ending affiliated baseball in more small towns across America.