The 50% is simple math. If your velocity is V, you take V estimation points of work into the Sprint. V is a stochastic variable with a mean and a variance. You rarely deliver exactly V points: it is just the mean. About half the Sprints finish early, and about half the Sprints don't deliver everything on the Sprint backlog.

So this isn't about failure at all: a failed Sprint is one that fails to meet the Sprint goal. I suggest taking a Scrum class to learn how velocity works, how agile works, and to learn how to manage uncertainty (and if you don't admit to this 50% uncertainty, then you are lying to yourself and cooking the books) and to learn about Sprint Goals.

Your post suggests that you understand none of these things. They are all Scrum basics.