I view this as addition by subtraction for GSW. Yes, he was an important part of their championship this year, but he will be 30 next year and will start to slide soon. Meanwhile they have Kuminga and Moody who need minutes. I would wager good money that BOTH of those guys will be better than GPII as soon as next year. By the end of that 3 year deal, they will be MUCH better than GPII. I think this is a reach for Portland honestly.
The most impressive thing GSW has done is have the confidence and patience to hold on to their young talent and not trade them for "stars", trying to extend the title window for their core. Generally (outside of KD), they take on distressed assets, rehabilitate them, and trade them for assets or let them walk before they get too expensive. It always amazes me when teams trade their cheap young talent for aging, past their prime players like Deandre Jordan. It almost never works out.