I think if Singleton wants to go to the NFL ASAP, he won't find a better situation than what he has here. He'll have a fully healthy King back, in an offense that's going to probably make him the centerpiece of our passing game, and he'll know our playbook inside out. The chemistry he'll have with King, who will be a top 5-10 QB in college football, won't be easily replicated anywhere else. Key and Faulkner have shown they are finding creative ways to put the ball in his hands.
If he transfers, he risks getting stuck behind WRs with established chemistry with their QBs. You saw a good example of this with Philo and Stockton...every QB has "their" WR they know and trust. Singleton will have to fight for that at other places. Also, he won't be as comfortable with their playbook and his development will be a work in progress in his "WR for hire" year at the next place.
I guess that's the calculus Singleton will have to figure out. The chance to be a feature WR with a QB he has chemistry with versus the unknown at another school that may set back his draft stock. He may make a few hundred thousand more at the other school, but he could be risking more than that with his NFL prospects.