There is a "flat" mode that does what you want. Look to the upper right of any post for the view choice.  
(view: tree/flat)

Click on flat you will see the format change.

There is a problem with this format here on TER though. With flat mode all posts are in chronological order. Unless a poster quotes the post they are responding to it's difficult to tell who is responding to who. There are often misunderstandings when someone thinks a response is to them but it's really to someone else completely. It usually happens with long threads where a seperate discussion branches off. Example: Someone could responde to this post by me. Then two others could respond to you directly. In tree mode it's easy to follow but in flat mode the post of someone replying to me later will be at the bottom of the discussion and will appear to be a resonse to the one immediately above.

The flat mode works well for the Photo Board and any discussions where all replys are to the OP.

...this way for 7 years I've been around I'm thinking it won't be fixed. One other thing about this is TER is so large (members) it may not be feasible to use flat mode exclusively. It seems to work best on smaller boards. There are a few I'm a member of that use it.

TER committed to the current forum software years ago. I don't know it's home-grown (written by TER programmers) or one of the open software options.  

As almost everyone agrees, it's not the best choice for what the forums have become but we make do.  

Has anyone out there ever converted from one software to another without losing the archive or breaking the whole thing