"It is common sense to keep your insurance policy for as long as possible" is 100% accurate.
Arguing that someone will pay you more than the value you oerceive it to be worth is a separate point.
Also the opportunity cost (which none of us really know what it was) is not lost, it is delayed, again because the value that is worth more than keeping him will change dramatically if Brady suffers a serious injury or set back in the next 9 months.
The insurance value of garapollo to the patriots is probably higher than the starter value is to the browns.