What a crock.
You really think the Sox were unaware of Mookie's talent? After his 2014 partial season, his average WAR for five seasons was freaking 8--for a total WAR of 40. Only Mike Trout was better for the same five seasons at 45 cumulative WAR.
The Sox with Betts were good--including the Sox best season ever, 2018--but guess what? One freaking player can't do it all.
So in 2015 with Mookie playing full time, the Sox were dead last in the AL East. The next two years, 2016 and 2017, with the additions of Price and Kimbrel--and Sale in 2017--the Sox won the AL East and were promptly slaughtered both years in the ALDS. To get over the top, DD brought in DH JD Martinez, whose OPS in 2018 was over 1.000. I believe JDM made everyone else in the lineup better, including MVP Mookie Betts. He also brought in Eovaldi in 2018.
Nevertheless, the next year, 2019, with Mookie having another good year and everyone else back from the 2018 juggernaut, the Sox didn't even make the postseason because the pitching collapsed. And Mookie can't pitch.
As for Mike Trout, the Angels haven't done diddly with a sure HOF centerfielder. They got to the postseason once, 2014, and lost 3 straight to Kansas City in the ALDS.
By the end of the 2019 season it was obvious the Sox needed to rebuild, and that isn't what DD does. Indeed, three of his big hires--Price, Sale, and Eovaldi--haven't done much for the Sox since 2018 even though the Sox have paid them a lot of money.
Thus letting Mookie go to the Dodgers was essential to have any hope of keeping Bogey and Devers--to say nothing of rebuilding the pitching staff.