My opinion on UX:
Any object of the same element-class should always follow the same working principle and look identical, so that the functioning of it is recognizable by users.
And here the “Reply-mode-selector” is actually a select-button, which looks like an input field and does not look like the select buttons in other places of the UI e.g.
These points where already mentioned but went lost in the long run of implementation. Thanks to pointing me to the earlier discussions.
On the general discussion of the feature “Reply as Linked Topic”:
Use cases for “Reply as Linked Topic”
Actually it’s all about not getting an otherwise healthy discussion stopped by 2-3 users starting their off-topic discussions in it.
User picking multiple quotes of older posts in the topic:
User tries to make a point on specifically selected statements which might get the discussion off-topic.
New reply on cold topic with long time of no replies:
This often results in slightly off-topic content which is better placed in a new topic.