I did a lot of research on launch angles the last weeks.
The best hit in baseball is a home run and the best angle to hit homers is just under 30 degrees.
And still if you look at the wRC+ leaderboard the average of the top 10 this year is around 15 degrees. Trout is a little over 13, harper at 12, judge at 13, I think only sano and alonso are above 20.
But why is the best angle 15 and not 20?
1. Mlb players are not able to hit consistently under the ball even though they preach backspin. Due to the round bat it is more of a bell curve with just as many hit under and hit over.
From 1. Follows that a guy who averages 30 degrees has half of his balls above 30 degrees and thus useless while the 15 degree guy will have more of his hard hit balls between 10-30.
Also more uppercut than 10 degrees means that you are not as long in the zone and thus less contact. 15 degree attack angle might still be ok but at 20 you will miss more. So to hit consistently at 30 degrees you either need to swing under the ball which is first not possible against good pitching and second means less exit velo due to more spin.
For certain low contact power hitters it might make sense to sacrifice contact quantity and babip to get more power by swinging up more (ryan schimpf, joey gallo) but for most it won't. Guys like miggy and trout hit already enough HRs with their 13 degree average.
I think most guys below 10 benefit from increasing their LA but above 15 the returns are dimishing.
And last at 15 degrees any ball from 75-100 mph is a hit while at 20 there is that donut hole with 75-85 being bloopers, up to low 90s mostly flyouts and then again extra basehits in mid to upper 90s meaning that higher angles are more velo sensitive.
In short: hitters should aim to hit knuckle balls at 15 degrees and enough good stuff will happen.