For Dortch, people made the argument, "Well its not like he was going to get any taller," for why he left after his RS-sophmore year, for Sage I've seen people say its not like he was going to get faster, which isn't exactly true, but Sage could have put more tape out there showing his ability to get separation. Guys that leave that early are not allowing themselves the benefit of showing their evolution through the seasons. More games, more tape, less question marks. Seriously though, Sage was a RS-Sophomore, he could have come back and played 3 more seasons if he wanted. Of course he wasn't going to, but no one forced him to enter the draft this year.
And I know I am going to get shit for this, but how do you take a whole season off to prepare for a pro-day and not get a better 40 time? Mike Mamula showed you could hack the combine in an off-season.