Agree with you. Almost half the tread is gone. I'd replace both sides at that point. Tires seem to be under appreciated. I see all these SUV ads showing drivers blissfully blasting through snow and ice on all season tires, realizing that in real life, that would be pretty stupid. Same goes for running on old, dry rotted or petrified tires. Tires have a finite life, regardless of how many miles they have. A flat at speed is no fun. Consider the death of Paul Walker:I realize this. Do the math though. That is about a quarter inch difference in diameter.
I know that this isn’t the actual tire size, but for instance, a new 23” (72.22” circumference) tire vs a used 22.75” (71.435” circumference) tire is a difference in circumference of .785. Which is actually pretty significant. Also, the used tire should have been measured at the highest wear point, because that is the real effective diameter. The difference is likely much more.