The 'market' usually does a sufficient job in determining the short, medium and long term value of a given thread.
* if enough people respond to a thread keeping it on page 1 or 2, that is a solid indication that the OP or the responses are relevant/compelling/interesting enough to allow it to remain unmerged
* at some point, the thread will lose its momentum at which point, it will fall off of page 1 or 2 and then moderator discretion can kick in about merging into appropriate mega-thread.
* agree that mega-threads should be truncated to include only 12-18 months of posts, with rest of posts being searchable, but archived separately.