Except it isn’t, as you are the creator of the software in question, and you have designed the software so that it enforces that opinion. If it were up to our community, this would have already been changed.
Sure, sometimes it is clear who is being replied to. But it is often not. And, since the poster had to specifically choose to quote the other poster, and said quote is only removed after submission, the poster may not be aware that it is not clear who they are replying to.
Shortening quotes is great, but it’s not always possible. Sometimes the quote is already short. Even if it’s more than one sentence, all of the sentences may be needed for the quote to make sense.
There’s also a clear exploit. A user can click the “reply” button for the topic, yet reply to the other poster. The poster will not be notified of this reply. Yet, to all readers, there is no indication that the poster did not click the “reply to post” button. This could be unintentional, but it can also be deliberate. Either way, the person being replied to is not informed of the reply.
It would just be easier and not cause problems at all if the person you are replying to is always indicated in the top right of the post. It’s unobtrusive, takes up no new space, and prevents all of these issues.
To put this in software design terms: You can argue these are all edge cases. But part of software review is to find the edge cases. In well-designed software, it is not expected for users to have to work around those edge cases, but for the software to handle it gracefully. When reporting bugs, the fact said bug has a workaround doesn’t make it not a bug.
Discourse is decently designed in general. But it makes sense that, with software that is constantly being improved over time, a new, large community with tons of experience with message boards might find some edge cases you missed.