Everybody does what they do (RBSinTo's Law), but in my opinion buying cameras as investments make as much sense as buying toasters as investments.
In my opinion they're tools for taking photos or making toast and nothing more.
Well put. The comparison to toasters is apt.
In the 1960s, the Diana was a toy camera, given away as promotional items or sold very cheap. Some forty years later, LOMO was in vogue, and I saw them selling for $100 on eBay. I bought one for $7 and a friend was amazed that I scored such a deal. Fast forward to now, and they're down in the $10 to $20 range for a used one.
Cameras have no intrinsic value. Mass market cameras are by definition not rare. When the utility of the camera is factored out, what is left? Compare the value of the old folding cameras that take 116 film, or 828 film to those which take 120 film or 35mm film. The big notable exception is the Kodak Bantam Special, which is a beautiful and not plentiful camera (worth $100-$200). Compare the Univex Mercury (35mm in proprietary film spools) to the Mercury II (35mm in conventional spools). The Mercury II is worth more because it's still useful.
What will the useful cameras be worth when they are no longer useful as cameras (film is no longer made)? The answer is hard to predict across the board, but I think we've seen what has happened now that digital is popular. The only cameras that have retained value were already rare, or professional cameras like medium format rangefinders and slrs, Leicas, Alpas, and specialized cameras like Linhofs, Horseman, Goersi, etc.) The once-prized 35mm SLR can be bought by the sackful.
Look at the rare camera auction sites for an idea of what camera would be a good investment.
Personally, I'd go for the submini TLRs like the GemFlex since they're rather rare already, and a dead (enough) format, plus they're small.
Finally, as we know, some cameras have become fashionable for awhile, skyrocketing in value, then settling down. It would be vital for the investor to recognize a good opportunity to sell. An uninterested heir would probably not get a good return because they would be liquidating at the wrong time.