No matter how tight a labor market is, there are always qualified people available if the compensation is fair. Companies that recruit poorly tend to train poorly as well. Employers often get caught up in a vicious circle. They recruit poorly, then fail to allocate sufficient training resources. Employees' performance suffers, their satisfaction levels are low, and they leave their positions, giving their employers the opportunity to start the cycle again.
Employers ought to treat employees as an investment rather than an expense.
But you cannot put 10 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 do a lot of things poorly.