You don't bother thinking about that. Over time you have a large enough sample size for attrition rate and you hire based off that. it doesn't matter if a few leave, a few will always leave.
It costs time and value when you really can't control it.
No matter what there will be attrition for reasons completely outside of a company's control. You find a comfortable attrition rate for your business. if it's too high you deal with it.