Stifle Tower
Punch Bowl Re-Filler
He has essentially been an Asst. Coach his last 3 years in the NBA. I know it's still a leap, but I actually think he could do it. Pretty sure he has been preparing for this knowing he has wanted to coach for a while now.
Dudes like Jason Kidd seem like they just decided to coach on a whim when he realized he couldn't play anymore.
I would argue (but have nothing to back it up), that former PG's would likely make better coaches - on average - than any other position. They have to have a better understanding of the game than any other player. And I do see your point re: Watson. A couple of things in his favor are, as you mentioned, that he's been preparing for coaching for much of his career. Also, he's well-traveled so he's been exposed to a number of offensive and defensive schemes.
I'd rank Messina and some of the asst coaches (not named Boylen) ahead of him, but I could get on board with Watson. He'd could certainly rely on help from Hubie Brown and Sloan - that's like having the Library of Congress in your office. As long as he brought in experienced assistants, I believe Watson could be successful. On the other hand, the Millers might just say, "why didn't we just keep Ty and continue to let him learn and grow?"