This is the very first thing I think of when I consider why old-timers left--and I've thought about this before.  Some of the most interesting threads are from around 2010-2011, and last month I stumbled across what had to be the beginning of signature campaigns.  Somebody came up with the idea of renting out his signature space, and people were all saying it was a novel idea.  I don't know why I didn't bookmark that thread, but I didn't.

The other reason is that some of these people might have gotten really burned with Mt. Gox and gotten out completely.  It's not just legendaries who don't post here anymore.  There are tons of abandoned accounts that are low-ranked, who left well before I got here in March of 2015.  Mt. Gox was huge, and I can see how it could leave an extremely bitter taste in one's mouth if MG ran off with all your money.  That's my theory anyway.

One very interesting thing is how well people wrote in the early days of bitcointalk.  Comparing post quality in 2010 to today is astounding.  As clever a moneymaking mechanism as the signature campaign is, it was definitely a quality-killer for posts here once it sucked in the poor, non-English speaking folks (sorry, but it's true).  Anyway, for people who really like bitcointalk, reading posts from way back when is almost addicting.  It was like a different world.