Mark addresses this already a bit with:
And I’ll add to it:
The core problem: shared content, divergent mental models. The moment two users see the same topic structured differently, you break the shared reference that conversation depends on.
Reply-to works different: When User A (nested) writes “as the previous comment noted…”, User B (linear) sees no previous because it’s post that’s three positions up, or fifteen, depending on which branch collapsed. Which also means an author can no longer be sure of how their post is seen by others.
Permalinks and post numbers diverge: If post ordering depends on view mode, then /t/topic/123/47 either means different posts to different users, or means the same post but appears in wildly different positions. (Not sure how it’s implemented right now - either way, not ideal)
And I’m sure there are more issues.
it comes down to: a conversation is shared space, and to foster good communication should therefore be structured the same for everyone