Well, it also seemed that their only chance of extending him was if they first reset the tax penalties, which they could not do without trading Betts or other expensive and far less desirable contracts. So trading Betts was the smartest plan to reset, since no one was going to take Eovaldi, Sale or Price off their hands. (And they did try. I believe there were some Price/Wil Myers rumblings, which shows how far the Sox were willing to go.)
But the Dodgers were the only team willing to take Betts (and most of Price) and give up anything of value. My only problem with this unfortunately necessary trade was it did look better when the Sox got Brusdar Graterol...