Anecdotally, it seems like a newer trend that so many Juniors are declaring early. Max Olson has been following this story closely and reporting on the trends yearly, but this has only been the last few years. He writes for the Athletic now so I can't read his articles.I don't know the metrics well enough to know if this is a high % or low %. I would think if a full third of people didn't get drafted that's a pretty good showing.
Then again, I'd have to see historical data to be able to draw that conclusion.
Leaving early to me is really a question of your draft ceiling and your ability to stick with a team.
The most important thing in determining your instantaneous earnings is your draft position and it is really only significant if you go in the top 3 rounds.
The most important thing in determining your long term (5-6 years) earnings is your ability to stick with a team. With no guaranteed contracts getting drafted in round 5 and then cut does you no good. Being an UDFA who plays for 5-6 years can out-earn a 6th-7th round pick who plays the same amount.
So, if staying an additional year helps move you up the draft board (especially into the top 3 rounds) or helps you improve your chances to make and stay with a team - don't leave early. If you have already maxed out your abilities, it can make sense to make the move and start earning earlier.