I wouldn't pay him $15m a year. That's for sure. Like you say, Rubio gets paid $16m a year, and while Rubio is no star, he's a much more consistent performer than Lonzo; at least he can hit his free throws, which is the least you expect from a PG.
The way I see it, Lonzo is gonna get a big offer from someone. He still has all that background hype, his stans still exist swearing he's going to be Curry with Magic mixed through, his highlights still gather massive views, etc. He's a name. When his FA time comes, some desperate team will throw him at least $15m a year, and I don't think he's worth that. So unless we plan to just let him walk for that money, we trade.