I assume that the mods can already merge multiple threads on the same topic into a single mega-thread. The trick is to do it before the threads have too many posts so the merge retains coherence. One problem with lots of mega-threads is that they tend to dominate the front page so other threads can end up pushed off very quickly.
Same problem with just forcing people to stick to a single big thread. It increases the load on the server generally (1 thread with 10000 posts is worse than 100 threads with 100 posts) and newcomers would have to wade through a lot of potentially irrelevant pages or risk saying something that was already answered on page 946 when they only checked 948 to 978.
It's probably better to spin off certain groups of threads to a child board when there is a consistently high number of related threads.
One problem currently is that newbies are forced to basically post everything into the newbie section and once they start, the threads tend to stick there.