[account42]

The issue with having both linear chronological and tree views is that users of either still need to respect the conventions of the other. For linear views it is common to just reply at the end while tree view users will be a lot more tolerant of diverging discussions as those not interested can simply ignore that subtree.

Replying to multiple posts in one comment is also something that tree view discussion boards generally don't support - although it should be possible to do it.