I think it's pretty clear what's meant by it, and "hit by a bus" has very similar problems if taken too seriously. Were they hurt? Are they out for a few days, just shaken, decide to change their approach to life etc.
> How much did they win? Maybe they’ll still keep working? Maybe they’ll go off to start their own company and take a bunch of key employees with them? Maybe they’ll be broke in a year and come back to their job? Maybe they don’t leave right away?
Well exactly. You don't know, and now you've given yourself four scenarios to think through:
1. Employee remains.
2. Employee leaves forever, maybe with others.
3. Employee leaves for a while but would come back.
4. Employee leaves, but with some time to plan.
1 should not be a difficult thing to plan for, and 2-4 all ask the same major question of "what if they leave". There's perhaps a slight benefit of not having to say "well assuming you are dead..." - I know I've had some time this year where I was trying not to focus on a death.
The key point is the risk. If your answer to "what if X wins the lottery" is "well as long as they don't want to leave..." then you're already setting yourself up to answer the question it's intended to propose - what if they are no longer working for the company?