Duncan Robinson looks like he could easily be a decade-long League guy
When a guy like Korver is pushing 2 decades in the league anything's possible 
what do you mean a guy like Korver? He is one of the greatest shooters in NBA history. Anyone that is essentially a top 5 player in league history at a valuable skill they can play a long time.
You don't seriously have Korver in your top 5 shooters of all-time.
He is 4th all time in 3 pointers made (Allen, Miller, and Curry are ahead of him). He is 10th all time in 3 point percentage, but several players ahead of him are far less prolific. The players ahead of him are Steve Kerr, Hubert Davis, Drazen Petrovic, Seth Curry, Steph Curry, Jason Kapono, Tim Legler, Steve Novak, and Joe Harris. 4 times Korver led the league in 3 point percentage topping out at 53.6% which is the all time record (more than a full percentage point higher than Kerr).
So yeah, Kyle Korver is one of the 5 greatest shooters in history (and I'm really referring to 3 point shooting, but Korver is also a top 20 foul shooter all time as well). The only guys I'd clearly have ahead of him would be Steph, Durant, and Allen. I'd listen to arguments on Kerr, Klay, and Reggie, but that is it given both his percentage and the prolific nature he shot them. Now I do think if guys like Bird grew up today, they'd be on the list, but Larry played in an era where players just didn't utilize the 3 point shot like they do today (and people forget that Bird had 4 straight seasons where he shot under 29% from 3).
So yeah, Kyle Korver is a top 5 3 point shooter in history. I don't really think it is all that debatable either given his standing on the all time lists for both makes and percentage.