The solution there is to have a gradient of criticality to each individual workload. If you have 100% capacity of an individual assigned to contractually obligated work then you are under-resourced and have a problem. In my experience, there are always nice-to-haves, and a resource allocation plan should include a good amount of those goals at all levels of granularity to give overall flexibility. I realize that's hard to achieve, and maybe even impossible for some types of organizations, but to me it's the only way to operate without either causing massive burnout or paying people to sit around.