Warner was the backup though. Trent Green whom they had acquired to be the starter was injured in preseason. So yes, Warner started every game but he would not have if Green hadn't gotten injured prior to season beginning.
Certainly, no team would like to lose their starting QB ... but they have triumphed more than you would think. 7 out of 60 Superbowls is 11.6%. I was originally only remembering 3-4 times until I started researching.
This doesn't count how many times a backup has been able to make a playoff run or get to the SB & lose. I didn't do that research. My takeaway is that is good reason to put a lot of effort into your backup QB. The right teams with the right backup can still be very successful.