I am indulge in preparation since january 2016,but still to take my first prelims :D(so take any gyan with a pinch of salt)
Every year after mains result an environment of confusion starts floating in market. Most blame their optional, few blame upsc, few luck.
I am not denying the element of luck, but we shall find our loopholes.
In my opinion you will not fail mains if you satisfy following conditions.
1- completed optional decently.
2- have the ability to understand the essence of the question
3- have the ability to instantly mould the content as per the essence of the question
4- good writing speed