I think those are two different things.
Those giant threads do draw traffic, like you said. Relative to thread deletions, individual post deletions don't mean much.
I'm not certain that it still works like this, but it used to be that if the opening post of a thread is deleted, the entire thread is also deleted. If prolific OPs delete their giant threads, VS may eventually step in to stop the deletions and restore old versions with User 039872 usernames slapped on.
I don't follow ADVr much, so this may have already happened. VS has been recycling deleted posts since they took over by changing them to generic usernames, so I expect they'd do the same thing to revive OPs to resurrect entire, deleted threads. Even if the OP remained active, they no longer have control of their posts or threads once they receive the Username Treatment. Imagine how happy you'd be to have spent years developing extensive threads, only to be kicked out of them and lose all control of your own posts.