1. Most organizations have EM with one or another job title and many of them function very well. It is not a problem of specific role if something goes wrong. It’s often unfit people.
2. The role of EM is the role of people manager. This role is hard to delegate to anyone else: outsiders like HR will not know people well enough, assigning it to a product or project manager will overload those roles with functions unrelated to the primary job (great product managers are leaders without authority - it doesn’t mean they should take over their cross-functional teams). Principals are not managers by definition, they are people who have chosen different type of career not willing to do anything with interpersonal stuff. People management is a scope of work big enough to have a dedicated person for a team of 4-10 people, bigger teams will need more.
3. Engineering manager is a first point of contact for the business regarding feasibility and planning topics. While engineers work, managers talk. In theory a principal engineer can be such person, but not every team can and should have someone at this level. In fact, most should not - this is the role for the teams with over 50 people and it’s an org-wide role. If you have principal or architect in a team of 10, you are doing something wrong with the career path and probably overspending.