What should be clear at this point is that Ainge isn't building just for this year. Cousins has one year left on his deal. You're going to give up promising young players and a premium draft pick on the hope that a volatile player like Cousins will re-sign?
Like it or not, whether it's the right move or not, Ainge's primary focus is the long term. Which means Brown, Tatum, BRK18, LAL18, and maybe Smart. IMO, in his mind, everything else is relatively nominal. Don't forget Colangelo said the Celts tried for a year to get that LAL pick. Ainge must be super high on the '18 draft and it'll take something special to pry one of those picks away from him. One year of Demarcus Cousins doesn't qualify as something special.