I can appreciate that point, and it will be interesting to see the market for Betts if his 2020 stats in the NL are close to those of Yelich.
Look at the past two years (when all Yel broke out)... bWAR: 2. Betts 17.7 (10.9, 6.8), 5. Yeli 14.7 (7.6, 7.1). Yelich was the batting champ both seasons (Betts once), and had a slight edge in power and speed, with more HRs and steals.
Runs:1. Betts, 5. Yeli
BA: 2. Yelich, 4. Betts
OBP: 2. Yelich, 3. Betts
SLG: 3. Yelich, 7. Betts
OPS: 3. Yelich, 4. Betts
Obviously, Mookie has been the top right fielder in baseball the last half decade, but Yelich is solid out there, despite his dWAR (which some say stands for dubious WAR), having won a GG in Miami.
The comparisons are worthy, but for someone who has watched Red Sox games forever and knows how many positive ways Betts can impact the game, I'd still take Mookie. But at twice the price?