To make this more clear I'll post two example layouts

The existing layout looks pretty slick, but it is confusing. The wanted layout gets a slightly more ugly layout, but it is infact easier to understand. If the lines in the last layout is removed, and only the indentation remains, then it looks pretty slick too.

An other important distinction is that the last layout more clearly follows "who do I reply to" and not "who do I digress with", which makes it more obvious why there is a single reply field.

Just to make it clear, in the last layout the reply field will be a reply to the first post unless the user clicks on "reply" for the last post. You never reply to the last post unless you explicitly do so. That will keep the thread on the topic for the first post and not so easily wander off into the blue as often happen today.

This is almost certainly a duplicate, I've been complaining about this (although less articulately) for a while :(