Here's a different way to look at tire replacement. If you have a replacement point for your vehicle, aim for it needing tires at the time you dispose of it. For example: you keep your car 4 years. Buy new tires when it's two years old. Or, you want to trade at 100,000 miles. Replace the tires at 50,000 miles.
It finally hit me that wearing the tires down to 70,000 miles meant I "gave away" 20,000 miles of unused tread when I sold the car at 100k.
Replace the tires so you get the benefit, not the next owner.