Reasonable people will accomplish a reasonable amount in a reasonable time period. But, you cannot put ten quarts of water into a five-quart container. There is nothing wrong with shifting work from former employees to those who remain. Our capacity to produce will sometimes be enhanced as we take on more responsibilities. But there is a limit. Being truly overworked helps people to a lot of things poorly. It may be more productive to ask them to accomplish fewer things productively.
There are some who would say, “there’s good stress and then there’s bad stress”. That’s sort of like saying, “there are good heart attacks and then there are bad heart attacks”.