I had a problem with this as well, which has sometimes caused me to go to a 2 year sock cycle. Basically, once I see the first few I have to toss due to holes/threadbare I know the rest are at over 90% done, so no longer "perfectly good"
I disagree with ya there. I have some socks that are easily 20 years old that are still perfectly fine even if their litter mates are long gone. I wouldn't be surprised if I still have some socks from my college days.