There has been, what, 50 different TLDs now? Does this hold true for all TLDs? I mean no matter the TLD, would the price of each and every one of the domains go up in the long run?
I believe domain age one of the things a lot of SE's place value upon, so in theory an aged domain (with some sort of content on it preferably, and not a parked page) is worth more than the same domain registered 5 minutes ago.
Yes. As the availability of these domains dwindle, the value of exisiting domains will rise. That's in addition to any SEO benefit.
So may I put it this way: it's just like real estate business and the fact that real estate investment, even those from individual households are calculated in the gross domestic investments. Real estate properties never depreciate in the long run because it's becoming more scarce day by day.
It really depends on the domain. If you invest in the right one, the older it gets the more the value rises.
IMO the best thing about aged domains is the fact that they can avoid the google sandbox (that's for domains that have been used not necessarily parked domains)