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.  

 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.  

 I can see your point now 

 If kind 1 clients must support every other client just to display replies correctly, then we are making their life more difficult. 

It's not an issue for Amethyst because we implement everything, but it is a pain for any new dev. :(