Good points by both you and Notin--of course. I had no idea Sale's extension price was that small. So, if you're math is right, the Braves get 2 years of Sale for $22M--thanks to the Sox shelling out $17M this year--and can get a 3d year for $18M. I have pointed out (that's me patting myself on the back) that the Braves have yet to start Sale on 5 days rest, which is considered normal. Instead, 6 or 7 days rest. And that sure seems to be working.
So this was a good deal for Atlanta and, honestly, a pretty darn good one for Sale at age 35 with a checkered 2019-2023 in his resume. My guess is that no one--not Cora, not Breslow, not whatever "brain trust" still exists in Boston, and not JH himself--on the Sox thought Sale was being reasonable.
I have to add, however, that, thanks to the new pitching coach, the Sox rotation this year has been way, way better than expected.
Right now the Sox team ERA, 3.53, is 4th best in MLB and the best it's been since 2007, when it was 3.87, but ranked 2d in MLB. Indeed, if we buy moonslav's entirely credible thesis that, because the Sox defense is so awful this season--8 more errors than any other team--we can assume that some of the "earned runs" ain't really earned. So it's possible this is the best Sox pitching staff in, what, 50 years? The giant caveat there is this season is a long way from over.
So, while I admit the above is an exaggeration, letting Sale go--at least so far--has not been all that horrible.