As someone who owns quite a few posters valued at over 5K and 10K dating from before 1950, I can state that I have never really looked at my approach to the hobby as an investment, so the cyclical nature of the market doesn't really faze me. So here's my reasoning: movie posters is just one aspect of what I collect, along with Old Master paintings, drawings, rare maps and old books. I don't consider any of these categories as a money-making scheme, since I have never sold a single item, and don't really plan to at this point. I collect posters that speak to me on an aesthetic level, and the older ones (from the painterly style of the 1930s RKO productions to the exquisite lithos from France of the 1950s and 1960s by the likes of Noel and Soubie) are a great deal more appealing to me than anything done after the mid 1970s. That is the only reason why I buy the posters that I have, not because I'm trying to capitalize on any trend in this field.
To me, there are 2 things in life that I enjoy spending my money on: real estate (houses mainly) and art (fine art or popular art, such as movie posters). I try to make educated decisions on what I buy so as not to grossly overpay for anything, but at the end of the day, I go after things (or places, in the case of real estate) that I love. This criteria is obviously the driving force behind what most of us around here decide to collect, and it is by definition a highly subjective one: I respect Thierry's completist approach to collecting, but to me the notion of owning every single piece of paper available on a specific title is completely bonkers, as even the best examples are bound to offer a repetitive image at best (such is the case with Jaws for example), while other posters are simply not that good to begin with (at least imho). Therefore, it would be foolish for anyone to expect that we all agree on what constitutes an objective value for any given aspect of the field.
Besides, regardless of the surprising results of EMP's last auction, I still think the value of paper from earlier periods will hold up over time, because what is rare will always be appreciated by some collectors, no matter how small the niche.