I thought yours was a very well though out post. I would offer that there is one aspect of certainty: the capital stock of the United States, including population, is growing, and will continue to do so for the foreseeable future. This means that
over the long term,
the collective value of the U.S. stock market is to going go up:
http://upload.wikimedia.org/wikipedi...log%29.svg.png
This has been true since the foundation of the Dow, and is something that is sufficiently certain to justify the wisdom of investing in an index with a long term outlook.
There may be no certainty except death & taxes, but the benefits of dollar cost averaging and long term investing are as close to certain as you can get.
And yes, hope you don't hit 67 in late 2008. But if you do, don't panic.
When you are considering the implications of this given, also consider that there are more people and more money now than 10 years ago, and there will be more of both in 2025.
But the supply of air-cooled Porsches manufactured is fixed. It ain't going up. So supply is relatively inelastic.
Preferences may change, of course -- and interest in old cars may wane with time. But you are decades from seeing enough old geezers dying off that 60s-80s Porsches have the limited appeal of Model Ts. (and even '23 Model T coupes are still worth a suitcase of money.)
The foam may blow off the top of the market (as Matt Monsen describes) -- but there is a firm floor rising below the foam.
