The law cannot provide a remedy for all wrongs. The cause of action has been out there all along, in some kind of embryonic form. Let's just fess up and admit that we're dealing with a new cause of action here. Rules with lots of exceptions become confusing and unpredictable. The vaunted predictability of the existing categories is not endangered by reocognizing the new cause of action.
Decisions based on rules that are shot through with exceptions lose their legitimacy.
Like Frankenstein's monster, it may be difficult to control your creation. In emotional distress context, argument is that mental suffering is easy to feign and difficult to prove. The existing categories at least have the virtue of being able to sort out the wheat from the chaff. If you get too abstract, it's hard to come up with a legitimate boundary. As science advances, it is possible to introduce expert testimony to guide jurors' decisions about, e.g., whether mental suffering is genuine.