This whole thing is a good reminder of why signing Horford in free agency was such a giant leap forward for this organization.
Danny (reportedly) could have traded for a half season of Horford at last year's deadline. Instead he rolled the dice that he could sign him in free agency and won, keeping our stock of assets intact. In the process, the Horford signing (and the Durant meeting) solidified the narrative that the Celtics are a legit place for free agents.
Why give up a boatload of assets for Westbrook now? You're not a desperate team that has no shot at free agents; you just proved that. Might as well be the best team you can be this year, continue to grow the team's appeal, and roll the dice again in a year. Keep in mind there will be a handful of super FAs available next offseason, not just Westbrook.