This underscores the fact that you can't tell the condition of trailer tires by looking at them. Yes, in some cases, the sidewall will crack like a desert floor, but that's few and far between.
Trailer tires will wear out LONG before the tread will wear down. The tires will look fine, but the rubber will deteriorate over time to the extent a failure is imminent.
Trailer tires have a life expectancy of 5-7 years. Sunlight will hasten the degradation process.
Therefore, it's absolutely critical you change your tires every 5-7 years. Shorter if your boat is stored outside in direct sunlight, longer if stored in the garage.
A good protectant with 100% UV protection will help extend the life expectancy of your tires. BTS provides that kind of protection for you.