Mostly because we already use other kinds to be replies in other nips. Kind 1 replies are rarely used outside kind 1 clients. And they break many clients when they are used in other nips. So, it's really not something new that I am proposing. It's just a formalization of what already happens.
Discussion
Let's take long form content for example. Aren't the replies to that kind 1 events?
Yep, and many clients show the replies without showing the head article in a broken experience.