Thinking optimistically, I think the only way a missed milestone could be a positive would be when it reveals an issue. From what I gather, sometimes project managers don’t have total control over the deadlines of a project. Sometimes it’s dictated by a consumer, a sales manager, or a higher-up that may not have the insight into how long processes and tasks can take. Thus when milestones are missed, these people are forced to understand why this didn’t happen and learn from this mistake. Wishful thinking of course.