I read Buster's ESPN piece, and I didn't find it that outlandish. There is also a good contrasting piece (free to read) over at Fangraphs that concludes that the Sox are likely to get excess value from the contract (
https://blogs.fangraphs.com/chris-sale-finally-cashes-in/).
At the end of the day, this really comes down to how high you think Sale's injury risk is, and how much you think a generic pitcher aging curve can be applied to any particular pitcher (and whether or not you think such a curve is being adjusted fast enough to keep up with the modern game), and how you view the dollar value of a win. The Fangraphs approach is to take Sale's recent production, which is obviously super good, and apply a gentle decline from the aging curve they use, so that over six seasons he goes from about 6 to 4 WAR in a linear fashion. They also make some assumptions based on the value of a win (in a WAR sense) that is inherently backwards looking.
Buster differs in that he specifically sees Sale as an injury risk that is greater than would be implied for another random pitcher, based in large part on how he missed time last year and has previously seen a downturn late in the season. He also clearly views the normal aging curve skeptically based on some comps, though it's not clear that he isn't cherry-picking those while ignoring others, and seems to believe that any pitcher contract that covers the mid-thirties should be viewed with the assumption that the decline in performance will be non-linear.
There is a certain amount of grey area in this because how you view the contract (overpay vs underpay) simply from a team value standpoint necessitates predicting outcomes, and that's not a sure thing. While the projections Fangraphs is using make sense as a year-over-year performance estimate
assuming close to perfect health, would anyone really be surprised if Sale missed a few months (or most of a whole season) once over the life of this contract? Buster is also noting the more recent trend of contracts in baseball, and that we a.) don't see a ton of big extensions for mid- to late-career pitchers, and that b.) the ones we do see often don't work out. I don't believe that the Fangraphs $/WAR estimator makes a distinction between hitters and pitchers, and it assumes inflation but not deflation. It's just a matter of how much risk tolerance you want to carry.
I admit, I don't love the idea that the Sox have two of the largest pitcher contracts in the game for two guys over 30 (Sale will be any day now), while not having their position players locked up, but that's the price you pay when you fail to develop/keep home grown ace pitching.