If they retained Mookie, who does it stop you from signing? Not Arroyo. They wouldn't have Wong, Verdugo and maybe Eovaldi. However, picking Mookie's worst professional season is definitely one way to go about the argument, especially when he was still worth almost 4 fWAR. In 2019, he was already making 20M, so what does that additional 10M stop the Sox from getting? Just Kiké? So you're really comparing Mookie's RF replacements and Kiké to Mookie. Over the following decade, I think the Sox would have been better off having Betts as a fixture in RF than the uncertainty that came with turning over the roster.
Or is the real consideration that Henry can only willingly "afford" one max contract. Is the choice between Xander, Raffy and Mookie and which one do you pick? They went with the youngest of the three. We'll see if that choice was the correct one. So far, it's not looking good.