I agree with Craig,

older messages should not be removed !

But I like my earlier suggestion, that new postings can bring older messages on the top again. Then not a new thread must be created and all relevant messages can be read on a single thread.
This can cause, that threads can growing big, so a reverse sort inside the thread can be better (newest posting on the top).

E.g. some time it occurs, that the opinion on a topic was changed during the last year of practice and so it can be append to the thread and have also a chance to be read.

To bring a threat on the top again, can also avoid, that new posting overlooked.
Now this often occur, since all read lights are yellow on returning to the message board after reading the first new posting.


Peter