If they are that critical then they are an unrecognized partner. You could consider giving them a cut of the business. If you don't want to do that, then I th

If they are that critical then they are an unrecognized partner. You could consider giving them a cut of the business. If you don't want to do that, then I think you need to start finding a way to spread their responsibilities around to less critical people who will have no ownership of the business.