Actually it is a performance issue and in the way it is implemented now it is only possible with a depth of 10 replies. But I have no technical problem to increase it to a fixed number and I will do that.
The issue is to find a fast algorithm that can in one database-query figure out the complete hierarchy of a message-thread; it is a bit tricky to do that efficiently in one SELECT SQL statement because messages can appear in any order and can be a reply to any message. The issue is in the sorting of the messages to get them returned in the right order.
Its a completely different issue in other boards where you only post a message at the end of the reply chain (flat view boards).
But I will improve the current boards specification when I start digging in the messageboard architecture.