They are also replaceable cheaply. Also agree on the wider point that he'll play this contract and that's likely it, but they should keep him through the year as he can get hot. Not what you want from a RB (microwavable vs steady heat) drafted in the first round, but at the same time draft inexact science being what it is at least the Pats got the full contract and production especially in the playoffs a couple years ago versus a full bust (1-2 years and out).
Outside of the injuries I think Sony gave the Pats exactly what they wanted. He missed three games to start 2018. If Sony doesn't get hurt in 2018 and you project his regular season stats out over 16 games he would have rushed for roughly 1217 yards and 8 TD's at 4.5 yards per carry. 1217 yards would have garnered him the 4th most rushing yards in the NFL in 2018. He led the entire NFL postseason in yards, yards per carry and TD's.
In 2019 their blocking was horrid, they had zero TE's for the first third of the season due to injuries and Watson's suspension. The O-Line suffered some injuries and finally Develin's career ended. By the final third of 2019 Sony's numbers were much better in concert with with better blocking. That team failed because they couldn't pass well.
Sony's running mate at Georgia didn't play his entire rookie season so it would be unfair to compare, but in 2019 Chubb played 68% of the Browns offensive snaps on the season, Sony played 37%... about half as much as Chubb on offense. Chubb rarely left the field, the Patriots rotate players in, they use players situationally.
The Patriots don't lean entirely on one superstar back, and even if they drafted one he likely wouldn't see a second contract in New England. This is the same team that let Chandler Jones, the best pass rusher in the league, leave for a 2nd round pick because he wanted 100 million dollars. At some point players become too expensive and some positions (QB, CB, OL) are more important to spend your cap dollars on.
Some fans believe fantasy football success = real world football success. It doesn't, football is won by the best team in three phases. It's not who scores the most points, it's who wins the point differential battle.