Some of this is a little too specific to deny any parallel. Nomar was on a fast track to the Hall of Fame and one of the best shortstops in team history before injuries derailed him, but he was a better player than Cabrera. He was worth 5.8 fWAR in 2003 a year prior to the trade. And while he was only worth 1.5 fWAR in 2004, as the injuries piled up, he was still worth more than Cabrera that year.
Whether or not Cabrera and Mientikiwicz were prospects is immaterial. It's not like the Sox have swapped Betts for prospects yet. They did swap Edgar Renteria for BA #2 Andy Marte, but then flipped Marte for Coco Crisp and others. No rule against doing that again.
As for dealing for a star for prospects, the AGon trade leaps to mind. But even if you don't like that example, just because the Sox have never done it before is no reason to not do it now. After all, when was the last time the Sox were trying to pare salary to get under the luxury tax limits? The situation itself is novel, and therefore the solution might need to be, too.
No one wants to white flag 2020, but this team will still have Devers, Bogaerts, Benntendi and Martinez near the top of the order. While Mookie does make it even better, let's not pretend this lineup is built around Cody Ross without him. With or without Betts, the Sox will score. The season largely rests on the health of Price, Sale, and Eovaldi. And if they all go down, it won't matter whether or not the Sox have Betts...