About in late 80s scouting got a lot better. In the first 20 years of the draft era for example there were tons of busts among first overall picks and hardly one star player. Then in the early 90s a few first overall picks became superstars. There still is an occasional bust like Mark apple but most first overall picks now at least become an average player if not more if they don't get hurt.
The player development thing is also real. 40 years ago many HS picks were like 150pounds and have never lifted weights, nowadays HS picks start lifting at age 13 and weigh 180-190 and they also have hitting and pitching coaches and might even do like weighted ball training so they throw 90+ in HS. This removes some of the uncertainty and the need to project growth.
Players nowadays are better at 18-19 but also improve less at 20-23 because they are pushed earlier so it is easier to judge for scouts then guessing whether the 155 pound kid from 1970 will add 30 pounds and add 10 mph to his fastball.
In fact many top HS pitchers don't improve their top velo at all in pro ball. They are still getting stronger but due to pitching more their top velo doesn't even increase or sometimes even decrease.