I would suggest that we think of realistic in terms of confidence levels. No matter how loudly we declare and defend them, plans are simply variously sized and shaped boxes of assumptions. The more realistic a plan, the higher levels of confidence we have in its assumptions.
We consider plans more unrealistic when we have lower levels of confidence in them. One of the factors in confidence is the presence of questions in the form of intentions, unknowns and concerns. The more these exist, the lower our confidence level. We can increase confidence levels as we reduce them through conversation and action. We can also increase confidence levels by pushing them out further with longer completion timestamps.