Charles, this issue comes up fairly frequently. It simply comes down to the reality that a strings player cannot produce a loud sound using a single bowstroke for an extended period of time.
Having said that, a player can very possibly sustain a moderate volume note longer than some of the sustains (as a bass player I can speak about the bass sustains first hand). If you own VI Pro, there is a very reasonable solution. The sustains can be lengthened by using the stretch tool (the tutorials note that stretching works best with non-vibrato samples, but one certainly can stretch samples with vibrato). IMO, the stretched samples match or slightly exceed what a live player is capable of, and can prove very useful in certain situations.