If you think about it, if Silverstream decided not the save Graystripe from the frozen stream, the series would have taken a different turn.

Another cat would have taken Silverstream's heart and then Feathertail and Stormfur would have been fully RiverClan cats, if they were even born.

If they werent, then who would take Brook's heart, and save the Tribe from Sharptooth?