Some people like to talk about the best case scenario to complete their projects. Some people like the opposite, and they always refer to the worst case.
I think there's a better way.
The likelihood of a best case scenario happening is usually very low. You are calling it "best case" in the first place because you know something could (probably will) happen. So you make sure to label it as "best case" to properly set expectations and let people know that it won't likely happen.
If it won't happen, why do you talk about it?
The worst case scenario occurs when multiple things go wrong at the same time. The interesting thing here is that, when too many things don't happen the way they should it becomes hard to determine how they will impact the timeline, thus the worst case scenario is usually a fictitious date far enough in the future to put everyone's mind at ease. Perfect storms could happen, but our plans for them will likely be insufficient.
Again, if it won't happen, why do you talk about it?
Giving best and worst case scenarios is an easy way to dodge the real solution to the problem: a realistic plan that accounts for risks and dependencies, and covers multiple possible situations and their outcome.
Coming up with this realistic plan is hard. You can't just hide behind a date far enough in the future, nor behind the unlikely perfect picture. You have to think hard enough about the work, dependencies, and risks, connecting all of these and determining their impact and how to mitigate them.
Whenever you've done your job, you'll feel confident standing behind your plan. Whenever you haven't, you'll probably try to masquerade it behind unrealistic expectations.