I don't agree with the premise that he's destined for the Lakers (just haven't seen enough evidence on this, though I keep hearing people repeat it). But with that said, I strongly agree with the conclusion.
I wouldn't give up any one of Jaylen Brown, the 2017 pick, or the 2018 Brooklyn pick for Westbrook.
He's got one year left on his deal and it's a good chance that he leaves a Durant-less OKC (assuming they keep him). You have at least a shot to sign him outright next summer. Remember: We were one of the only teams that Durant took a meeting with, and signed arguably the 2nd-best free agent on the market. Whether or not you like to admit it, Boston is now one of the most attractive destinations for free agents in the NBA.
The only way you give up one of those assets for Westbrook is if you think you can compete for a title this year. And that's such a gamble, given the talent in Golden State, nevermind Cleveland, San Antonio, etc.
Lastly, I. Thomas and Westbrook couldn't play in the same backcourt because they'd be severely subpar defensively, and they both dominate the ball. You'd have to ship IT out with the same trade, if you ask me. And are you willing to do that given the fact you sent IT to the draft lottery, made him the head pitch man in all your free agent efforts, and have otherwise made the face of your franchise? That would not exactly be a good look.
In other words... I don't see the value.