No doubt that when Coach K decides to call it a career, it'll be impossible to replace him. It was tough at UCLA, at UNC and Indiana and anywhere a phenomenal coach leaves. However, Duke does have an advantage that none of the rest had: Coach K.
He said at one point that while he would certainly advise the AD, presumably Kevin White, it wasn't his job to hire his replacement.
Given the way he thinks things through, we're sure he'll have given this a lot of thought as well and will probably carefully consider what happened at the other schools and why.
No one can precisely replace him, but the situation can be managed and the next guy, whoever it ends up being, can be given the highest possible chance of success.